Improve tick-frequency calculation to handle small charts

On some small charts, the tick frequency can be low enough that labels end up overlapping each other.

E.g. The genreCount chart on the front page when the width is initialised as 300px.

In this case, the tick frequency should scale to display ticks each 500 instead of 200.
I found this code confusing at first but I think it is the name "increaseTickStep" that caused the confusion.  I think the function returns the next tick value so how about renaming it to "nextTickValue"?