Bar chart (x,y) display on mouse-over

[jbudd]

When you hover over a bar the x and y value for that bar are displayed in a tooltip.

1. When the y value for a bar is 0 there is no way to get the (x,y) tooltip. A very small value results in a vanishingly small mouse-over area.
2. If any bar's value is outside of the y axis displayed range, no tooltip is shown.