Tune "Average time to solve"

[kelson42]

• Title should be "Time to solve"
• Y axis legend should "Average tickets lifetime (days)"
• Colors should be like in chart "Backlog size"
• Same bars as "Backlog size" should be used".

[florentk]

@kelson42 ok, but a same bar as "backlog size" has no sense because the sum of two average values is not a average value.

[kelson42]

@florentk Seems to be a pertinent remark. We should use curve then?

[florentk]

done via 11d22d16dffc633cae0b51d88ec3b2113b3d2d76