Remove 'flat spot' at end of line series

[simonhac]

we add in an extra point at the end of our series, which has the unfortunate result of creating flat-spots at the end of line and smoothed line representations:

i've seen people misinterpret this as 'no growth' — it'd be good to remove this ambiguity.

it's particularly noticeable in the new compare view:

[simonhac]

note how excel does this — column chart vs bar chart:

[simonhac]

note also that excel has an option of where the axis crosses — on the tick marks, or between them. i'm not sure which is better.

if you choose 'on tick marks', the x-axis labels need to hang off the right-hand side of the chart, which is not necessarily bad: