The report module outputs both text based reports in a table and a graph. Why stop the module from working if the Google Chart API isn't installed? The text reports as they stand are infinitely more useful than the graphs and you can't view them at all without having the graphs as well. This also prevents you from generating a csv file which of course cannot have a graph in it. The logic should be that if the Chart module is installed, display a graph but if not just display the text report.

On a related note, the graph generated on the overview report is virtually useless as it graphs a maximum of 5 ads independently and there is no option to choose which 5 if you have more than that. It looks like it was added for the sake of having a shiny graph rather than presenting any useful information. A far more useful graph would be one which displays summed views and clicks for all the ads in the text overview report above.