Provide new scaling mode (VAL_AUTOSCALE_VISIBLE_DATA) for SetAxisScalingMode

[Wolfgang]

At present, autoscaling works with respect to the full provided data set, i.e. if all data are shown on the graph.

If say the X axis is set to manual scaling such that only a subset of all data is plotted, autoscaling of the Y axis still considers all data even if they are not shown, see the discussion here.

Hence it is suggested to provide an additional auto scaling mode which considers only the data actually visible on the graph, say VAL_AUTOSCALE_VISIBLE_DATA, complementing the current VAL_AUTOSCALE which actually is a VAL_AUTOSCALE_ALL_DATA

[gdargaud]

I'd go further and ask for the possibility to auto-scale only one end of an axis. This way you can for example have always Y from 0 to whatever the max is. To do this currently I have to plot an invisible point at Y=0.