[Feature Request] Add command line options to for a fixed Y axis scale

[Klaws--]

Currently, the Y axis (roundtrip time, milliseconds) is scaled dynamically, both minimum and maximum coordinates. That is, actually, pretty nice, but required reading of the Y coordinates to figure out how many milliseconds a "peak ping" took (or a look at the top line for the "max ping").

Proposal: a command line parameter, or two, to specify a fixed Y axis coordinate system.

Example:
gping google.com -min 8 -max 50
Minimum Y coordinate: 8 ms, maximum Y coordinate: 50 ms.
Naturally, if a plotted point exceeds these Y coordinates, the Y coordinate of the point must be capped to stay within the visible range (albeit at the minimum/maximum bottom/top location), so the user can determine whether pings got too slow or too fast (assuming pings can ever get too fast...).

[fxnn]

My reasons why I find this proposal great:

• --max is very useful because 99% of the time the ping is below, say, 30ms. But then there's one single glitch, the Y axis scales to, say, 1000ms, and all of a sudden you can't differentiate the lower times anymore. I would might set it to --max 100, as anything above is simply the "far too much" category to me.
• --min is something I would always use as --min 0, because I find graphs that don't start at 0 confusing. It simply disturbs the intuition when looking at the graph.

[MoralCode]

This could also help distinguish a failure to ping (which seems to be plotted as the highest value on the graph) from an actual ping that took that long (see #409)