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.
It's not immediately obvious that startDate and endDate are in milliseconds.  I think renaming these to startDateMilliseconds and endDateMilliseconds would be better.