Tooltips - Make it easier to select the data point you are interested in
Rather than just simply choosing the data point closest to the x-value of the cursor when displaying a Tooltip please consider making things much more selective:
1) Take the y-value into account as well
2) Display the tooltip for several data points when they are too close to select individually with the mouse
3) Only consider data points for tooltip display in series/ranges that are selected/highlighted.
Getting the tooltip to display the data point your are interested in is especially problematic when using the Line Chart with multiple series and many data points where the X axis values are similar (but not identical) across the series.
I'm sure however that something similar must apply to other visualizations.