I recently had a requirement to create a heat map of the US States and set about exploring if there was an easier way than creating a QV scatter plot mapped over an image.
I found the Google Chart API. With some help from the QlikCommunity forum, I got a fairly pleasing result.
The Google Chart API is provided free by Google. You pass in an http request with parameters that describe the data and desired layout, and a chart image is returned. I won’t cover the details of the API, it’s well documented at http://code.google.com/apis/chart/. Rather, I’ll share my experience integrating it with QV.
Here’s a screenshot of my results. The map will update as selections are made in the Sales chart. The generated map is not clickable. It’s just a static image.
A working QVW of the above may be found in the Qlikview Cookbook at:
The map is a Straight Table with a single expression. The expression is the http://… string used to generate the map. The representation for the expression is set to “Image”. Thanks to Tom on the forum for showing me this technique.
The OnAnySelect document event is used to trigger a “showMap()” macro that creates the variables needed for the http string. In a production application, you would want to be more selective and use field level events on the fields relevant to the chart.
In addition to chart layout parameters, the http string contains two parameters that describe the data.
- chld= provides the list of states
- chd= provides the data values for the states
The States and Values are associated by ordinal position in the respective lists. I could not find a way to keep the lists in sync by using QV expressions alone. The solution was to use the Sales chart as my “data source”. The macro walks the rows of the table to build two variables — the State codes and the Sales values. Here’s a snippet of the macro code:
set obj = ActiveDocument.GetSheetObject("CH01")
' Collect the locations
locations = ""
for RowIter = 1 to obj.GetRowCount-1
locations = locations & obj.GetCell(RowIter,0).Text
ActiveDocument.GetVariable("vValues").setContent values, false
The encoding algorithm requires that the maximum value of the dataset be known to properly spread the individual values across the relative range. To determine the maxvalue in the macro, I use the QV evaluate() function to “callback” to the QV expression language.
Producing a chart with the Google API does have some downsides. The user must be connected to the internet and the chart will render slower than a native QV chart. It also does not provide for making selections in the chart and tooltip values like a QV chart does. But I found it to be a simple solution to my requirement. I hope that someday the QV product will provide regional maps as chart types.
Update October 3, 2008: Alistair on the QlikCommunity forum has posted an example of calling the Google Chart API without using macros, which I find to be the preferred method:
The next update to the Qlikview Cookbook will include the “macro-less” technique.