Enter measurement values in charts

As an alternative to entering values with the keyboard consider entering values by clicking on a point in a chart.
There are several possible benefits:
- avoid the somewhat clumsy keyboard interface
- put the value in context (show previously entered values, percentiles or dynamically calculated ranges of "allowed" values)
Potential problems:
- it still needs to be easy to enter a specific value
=> position values aligned with a grid with the required precision, allow zooming
=> show a (+) and (-) button next to the value to allow small (manual) adjustments