change over time - Increased margin required above line chart

[giacomobg]

Small but fiddly enough for it not to be worth it for me to sort out now.

svg for line chart needs enlarging at the top because numbers appear (too far?) above the line and can get cut off when they go off the svg.

might help to play with #keyunit margins - might have to change some position css like in dvc895 international comparisons map.