Enhancement proposal:
Add a "ruler" like tool to the plot window.

Many other scientific data viewing programs (for example ImageJ) have this, and it is the feature I miss most often in the matplotlib plot window.

More detailed description:

• When left-click-dragging the mouse, a line is temporarily drawn from the start to the end point.
• Movement can be restricted to horizontal/vertical lines with the Ctrl/Shift key.
• As the mouse is dragged, the status bar shows:
  • Total distance
  • Angle
  • Horizontal distance
  • Vertical distance

Use cases:

• Measure the distance between peaks in a spectrum, correlation between time series, etc.
• Manual period detection in time series.