Resizing chart width with window.

All chart widths will now shrink with the window up to their minimum width.

There are known issues with gridlines and target markers but these are bugs that need to be fixed in AxisGridline and MarkerSeries.
600 would make it too wide for some devices by default.  300x300 is a nice little box that most charts without too many bars should fit in.

400x300 would work nicely too, and probably be more readable.

Not too fussed about this, most people will set the dimensions explicitly.