Dataflash Log Graph X-axis "freeze" feature

#### Issue details
Dataflash Log Graph X-axis "freeze"

#### Version
New Feature request

#### Platform
[ X] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

It would make diagnosing issues much easier if it were possible to "freeze" the x-axis (Line number/time). Often it is necessary to use one parameter to identify the point of a specific issue and then look at other parameters that may give insight into what is concurring at that point. Currently the graph zooms all the way out whenever a new parameter is selected making it necessary to identify the point of interest again. Having the x-axis fixed and allowing adding new parameters and allowing only the y-axis to be zoomed would be very useful

Start at the Dataflash Log Graph implementation and reproduce the current behavior by zooming the x-axis, then selecting another parameter. The work is done when the line number/time range remains fixed while parameters are added and only the y-axis can be zoomed.

Written by the indexing model from the issue text.