Line chart range is truncated

[raboof]

When drawing the line chart for an account and selecting an explicit date range, the line graph is truncated: before the first and after the last transaction, only one day is shown.

It would be nicer to set the range of the chart to exactly the selected range (identified in #1471)

To reproduce:

• make ghci-web, :main -f examples/sample.journal:
• Click assets, select the two months enclosing the three central points.
• In the query field you see the span I selected, eg date:2008/4/30-2008/6/29.
• But the chart is shrunk to the points' span (6/1 to 6/3).