[BZ 1146266] - Metric graph is empty if high, low, and avg values are all the same for every time bucket within the selected time range - d3.scale.linear().domain() funcion excepts an array with >2 distinct elements. The global min and max were used here, but if all the values were the same it didn't work so I've changed it to [0.9 * min, 1.1 * max] if min == max.

(cherry picked from commit 265d3ea40d4644df258689de6bbee30ac3c6a3bb)
Signed-off-by: Libor Zoubek <lzoubek@redhat.com>