bug fix (?) for googlecharts

[markus]

Hi! I don't understand what I was doing but my change is only one character and it fixes a bug for me in the case when I have 3 labels on the x-axis. Please take a look at my commit.

[aderyabin]

Can you show changes on examples?

[markus]

With this code:
Gchart.line(:data => [[nil, 500, 425], [425, 600, nil]], :axis_with_labels => ['x'], :axis_labels => [[1, 2, 3]])
I expect a chart where the labels 1, 2 and 3 are evenly spaced on the x-axis.

[markus]

I pushed a test for it, but I don't know if my test makes sense because I think there are many ways to get evenly spaced labels.