Chart Error executing child request for ChartImg.axd

The chart would work on one page but not on the next. Turns out if the chart is initialized for the first time in a POST (i.e. a postback) the error is thrown because the handler is configured incorrectly. To fix the issue modify the httpHandler configuration that user LaptopHeaven referred to in this topic by adding the POST verb:

In handlers of web.config add the following code:
<add path="ChartImg.axd" verb="GET,HEAD,POST" type="System.Web.UI.DataVisualization.Charting.ChartHttpHandler, System.Web.DataVisualization, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false" />