Generating Excel Charts with MarkLogic

GIVE MY CREATION LIFE!and some fun with Formulas too…

This is another interesting one I see regularly: “How can I generate Excel Charts from MarkLogic Server?”

Charts are actually rendered from DrawingML found in the .xlsx package.  The DrawingML is embedded in SpreadsheetML, which is the Open XML format for Excel 2007/2010.

You don’t want to mess with DrawingML, as it’s a nasty frickin riddle, wrapped in an engima, inside a russian doll style matrix of insanity and pain.

Word, Excel and PowerPoint are producers and consumers of XML.  To some extent and to varying degrees, each of their respective XML formats can be understood and worked with in a relatively straightforward and reasonable way.  Sometimes though, the XML generated by these applications is really just a serialization of their object model and you’ll just waste a ton of time and find yourself in an extremely uncomfortable place (ed-like the back of a Volkswagen?) trying to figure the XML out when you don’t have to.  So let’s leave the DrawingML be. Capisci?

Think about it this way:  A chart in a workbook is tied to certain cell values in a worksheet.  When the cell values update, the chart dynamically updates.  At the end of the day, the DrawingML is just a snapshot of what the chart looked like based on the cell values when the Workbook was saved in Excel. (ed-Pivot tables work similarly in this way, but that’s a post for another day.)

Now let’s say we have a workbook containing a chart.  We know we can save our .xlsx to MarkLogic Server and have it automatically unzipped for us, its component XML parts made immediately available for search and re-use.   We can then update our extracted worksheets in the Server using XQuery.  Finally, we can re-zip the extracted workbook components back up and open the updated .xlsx into Excel.  Excel will automatically refresh its chart for us when it consumes the XML so we see the latest visualization of our chart based on the information we added to the worksheets.

5 Steps to Chart Freedom

Step 1

Create your chart in a workbook and drive it off of some cell values.  Note the cells and the name of the worksheet you’re driving your chart from. (example: Sheet1, cells: B2, B3, B4, etc.) I’ve provided a sample .xlsx here.

On Sheet1 we see download counts for a fictional company’s widgets for the month of September. The chart shows downloads for the widgets Foo, Bar, and Wumpus.  The chart columns correspond to cells B2, B3, and B4.

On Sheet2 we see sales counts for a fictional company’s widgets for each salesperson. The chart shows total sales for each salesperson.  The chart sections correspond to cells E2, E3, and E4.  Look closer and you’ll see that the cell values in column E driving the chart are actually the result of formulas; they are SUMs of all widgets for each salesperson row.  Note that the cells B6, C6, D6, and E6 all contain SUM formulas for their respective columns as well.

Step 2

Enable the Office OpenXML Extract and Status Change Handling CPF pipelines for your MarkLogic database so the .xlsx will automatically be unzipped when ingested into MarkLogic and its component parts made available for update.  Also insure you have the URI Lexicon enabled for your database. An example how to set this up can be found here.

Step 3

Save your .xlsx to MarkLogic. Once saved, the .xlsx is unzipped, and we can now manipulate it’s extracted XML component parts directly.  The idea is to save workbooks containing your charts as templates within MarkLogic and then update the extracted worksheet parts based on new information being saved to your database.

Step 4

Use the XQuery API that comes with the MarkLogic Toolkit for Excel to set the cell values for your chart in the extracted worksheet.  In particular, look at the function excel:set-cells() for updating worksheets.  Evaluate the following in CQ.

Note: you may need to update the code samples below to reflect your workbook and where you saved it in MarkLogic.

xquery version “1.0-ml”;

import module namespace excel=”http://marklogic.com/openxml/excel” at “/MarkLogic/openxml/spreadsheet-ml-support.xqy”;

let $doc1 := “/MySpreadsheet_xlsx_parts/xl/worksheets/sheet1.xml”
let $doc2 := “/MySpreadsheet_xlsx_parts/xl/worksheets/sheet2.xml”
let $sheet1:= fn:doc($doc1)/node()
let $sheet2 := fn:doc($doc2)/node()

let $cell1 := excel:cell(“B2”,120)
let $cell2 := excel:cell(“B3”,99)
let $cell3 := excel:cell(“B4”,456)

let $cell4 := excel:cell(“D3”,127)
let $cell5:= excel:cell(“E3″,(),”SUM(B3:D3)”)

return (xdmp:document-insert($doc1, excel:set-cells($sheet1,($cell1, $cell2, $cell3))),
                xdmp:document-insert($doc2, excel:set-cells($sheet2,($cell4, $cell5))))

In the code above, for Sheet1, we see that we use the excel:cell() constructor to create cells for B2, B3, and B4.  We set the values for these cells to new numbers. These numbers could be coming from the results of another query.  We update the worksheet, using excel:set-cells(), passing the function the sheet we want to update, as well as a sequence of cells we’d like added and/or updated on the referenced sheet.  Finally, we xdmp:document-insert() our updated document, overwriting the existing one with our updated worksheet.  Remember, Sheet1 just held the simple chart driven directly from the cell values.

With Sheet2, we again use excel:cell() to create cells for D3 and E3. Sheet2 is more interesting as the chart here is driven from cells that contain formulas. For E3, we create a cell using excel:cell(), setting the value of the cell to the empty sequence, () , and passing in the formula for the cell.  Again we excel:set-cells() to update our worksheet and xdmp:document-insert() to save our updated worksheet back to the Server.

Note on excel:cell(): This function creates a new cell, so if you wish to retain an existing formula for a cell before you update it in a worksheet, you can’t use the 2 argument excel:cell() function.  If you did that, you’d lose the formula for the cell in the worksheet when you overwrite the XML.  You must create the cell with the formula, as we did above for E3.  If this doesn’t work for you, you can always roll your own XQuery to update the cell values for worksheets containing formulas in a different way.

Note on Excel formulas: Unlike charts, cells containing formulas will not calculate and refresh automatically when you open the updated worksheet in Excel if those cells already contain values. The value of the cell within the XML for the worksheet is considered the cached value by Excel and will be displayed when the workbook is opened.  This is done for performance reasons, so formula heavy worksheets don’t take forever to open as they calculate the value for every cell containing a formula when a workbook is opened.  Formula calculation is postponed to avoid wait time when opening a workbook.  As a result of this though, you can create XML for a worksheet that when consumed by Excel, will result in a cell displaying the wrong results given its formula.

To get a formula to calculate the value for a cell when you open a workbook in Excel and insure the correct cell value is displayed, you need to set the value of the cell to nothing.  You can do this using excel:cell(), setting the value of the cell to the empty sequence: ().

For more information on the excel:* functions,  check out the XQuery API docs that come with the Toolkit for Excel.  There are a lot of functions available, all documented and with examples of usage.

Step 5

Zip up the updated .xlsx from it’s extracted component parts and open into Excel.  When you do this, it doesn’t matter what the DrawingML is.  Excel reads the cell values when it consumes the XML and will update the chart automatically.  The next time you save the workbook, the DrawingML is updated to reflect what the chart looks like based on the latest cell values. Evaluate the following in CQ.

xquery version “1.0-ml”;

let $directory := “/MySpreadsheet_xlsx_parts/”
let $uris := cts:uris(“”,”document”,cts:directory-query($directory,”infinity”))
let $parts := for $i in $uris let $x := fn:doc($i) return  $x

let $manifest := <parts xmlns=”xdmp:zip”>
                         {
                              for $i in $uris
                              let $dir := fn:substring-after($i,$directory)
                              let $part :=  <part>{$dir}</part>
                              return $part
                          }
                         </parts>

let $xlsx := xdmp:zip-create($manifest, $parts)
return xdmp:save(“C:\MyUpdatedSheet.xlsx”,$xlsx)

Open MyUpdatedSheet.xlsx into Excel.

BooYaa!  We update a few cells on Sheet1, and our chart automatically updates for us when we open the .xlsx into Excel.

Now take a look at Sheet2.  We updated D3 and set the value of E3 to (). Subsequently, the formula in E3 calculated its SUM formula when the workbook was opened.  Since the chart is driven from E2, E3, and E4, it updated properly as well.  WooHaa!

But take a closer look at cells D6 and E6.  They each contain SUM formulas for their columns, and they’re displaying the wrong values!  (ed-#fail) This is because we didn’t set their values to nothing.  Since the cells contained values in the XML for the worksheet, the cell formulas were not calculated when the workbook was opened and the cached value was displayed.  If you click on each of those cells, you’ll see the formula, click off of them, and the cells will recalculate and update with the correct values.

Bring Excel Workbooks to Life!

There's always another way.So the title was a bit misleading, as we don’t actually generate charts, so much as create the appropriate XML for Excel worksheets so that the Excel application will update and render charts for us when it consumes the XML.   But understanding a little bit of the SpreadsheetML format and how Excel behaves when consuming XML for charts and formulas, the doors open up to some very interesting possibilities.

The above examples are intentionally simple, but think about this…

Instead of a dead .xlsx, that sits lifeless on the filesystem and only alive when opened and active and being manipulated directly in Excel,  workbooks can now stay alive in the Server, constantly updated from complex queries being evaluated as new information is saved to the database.  These workbooks can then be dynamically zipped up when called upon to open in Excel and provide snapshot visualizations and results for a point in time.  Excel will consume the XML and can update charts and calculate formulas when opening this snapshot workbook, while the underlying, extracted workbook lives on and continues to be updated.

But this is just one way to use MarkLogic and Excel together.  There’s always another way…

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