Interesting Edge Conditions on VantagePoint Graphs
Today a developer stopped by to say that one of the graphing classes - the combo graph (bars and lines), had problems when you degenerated it to a bar graph (no lines), and to a line graph (no bars). I checked and he was right on the money. Now one might ask why this is important, but there are times you'd like to just code it up and not worry about the number of lines or bars and have the GUI widget smart enough to just do the right thing. So I dug into the code to see how hard it might be. Interestingly enough, and I probably should have seen this coming, one was a lot easier than the other.
In the VantagePoint scheme, the combo graph is a subclass of the bar graph (hint, hint) and so the first thing I tackled was the case where there were no lines at all. The problem I had was that when I saw that the user didn't specify any lines in the combo graph I simply did nothing, and that's not the best thing to do in this case. What I needed to do was to see that there were supposed to be no lines, and then set that in the graph. Pretty easy.
Then I tried to do the same kind of thing for the bars, and surprise! that didn't work. NullPointerExceptions deep within the VantagePoint code that had clearly undergone some kind of code obfuscation so that the classes and method names meant nothing to me. Finally, through trial and error I came to the idea that VantagePoint was simply not going to allow a combo graph to have no bars. (well.. duh!)
I changed tack and said "Hey! A combo graph without bars is a line graph!" and what I did was to have the applet code detect the 'lack of bars' condition and switch the graph to a line graph and then plot the lines on that graph. Bingo!
I suppose I should have seen the lack of bars on a combo (bar) graph as a non-starter for VantagePoint, but I assumed that if it had either type of variable it would be OK. That was a mistake. The bars are the driving variables and the lines are the 'decorations' on the base graph. Makes sense now, but finding it in the VantagePoint NullPointerException was not easy. But I got it.