Line Chart: Option to show dots for every data point

[alykat]

For data sets with irregular data (say, presidential approval ratings, where the polls are taken at uneven intervals) it is sometimes useful to show dots for the points where data exist.

The dots shouldn't be turned on by default -- most charts don't need them. The dots should be toggled via the spreadsheet. (See how territory visibility is handled in the state grid map template: https://docs.google.com/spreadsheets/d/12e3cNKWd1E2IHcDGN72URkbp7Yjb_3TbJrAxIgpYCTI/edit#gid=0)