Avoid attempt to divide by zero when x-axis is datetime

[komi1230]

When we use datetime type for x-axis and the data is in the same week, this crate crashes with saying panicked at 'attempt to divide by zero'.

This is caused by the calculation of the number of points in a week.
When all data is in a week, num_weeks() method returns 0 and results in panic.

close #190

[AaronErhardt]

Thanks!