minimum max altitude of 2000m

This will make the altitude profile of grounded flights look like they
are grounder, so they don't get confused with floaters. Beyond that, the
height autoscales based on max altitude.

Author: rossengeorgiev

js/tracker.js

@@ -585,6 +585,8 @@ function convert_time(text) {
 }
 
 function drawAltitudeProfile(c1, c2, alt_list, alt_max) {
+    alt_max = (alt_max < 2000) ? 2000 : alt_max;
+
     var ctx1 = c1.getContext("2d");
     var ctx2 = c2.getContext("2d");