More PyPlot updates from Gordon Williams:

More PyPlot updates from Gordon Williams:
   - Imported modules given leading underscore to name.
   - Added Cursor Line Tracking and User Point Labels.
   - Demo for Cursor Line Tracking and Point Labels.
   - Size of plot preview frame adjusted to show page better.
   - Added helper functions PositionUserToScreen and
     PositionScreenToUser in PlotCanvas.
   - Added functions GetClosestPoints (all curves) and GetClosestPoint
     (only closest curve) can be in either user coords or screen
     coords.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29984 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775