Target line in utility graphs

[Wieltje]

With respect to 'energy awareness', one of the things i really miss in the utility graphs is a 'target line' which shows me if daily usage is below or above a yearly target value. I think this could be implemented pretty easilly for someone who is familiar with the code.

To clarify how i see this: input fields under 'settings > meters/counters' named 'target annual usage', which is then (divided by 365) shown as a straight line in the week/month/year graphs for water/gas/electricity graphs, just like the trendlines.

Anyone who is willing to give this a try?