Chart run-times in minutes by weeks

I am trying to chart run-time averages in Excel taken on a weely basis . For example the 1st week the average run time is 12 minutes, 42 seconds (12:42 in the spreadsheet); the 2nd week is 11:00, the 3rd week 9:44, the 4th 10:50. My minimum and maximum are 0 and 20 minutes.

I can get a chart easily enough, but I can’t seem to get the value axis (y) formatted to go from 0 to 20 in 1-min. (major tick) and 1/2-min. increments.

Does anybody know how to do this, or am I trying to do the impossible?