Can you kindly share a pictorial representation or brief further with an example in JSFiddle so that we can understand your requirements better and help you out?

Do you mean to show gridlines on top of area chart? If so please take a look at this JSFiddle which shows adding striplines at every interval over axisY to show gridlines on top of columns without reducing the opacity of columns.

If not, can you kindly brief your requirements further, so that we can understand it better and help you out?

No,
Lets say we have rectangle on the graph where it starts at 12:00 and ends in 13:00 (1 hour) its top is at value 10.
So what is the area? 5 (1*10/2) this is the area value I want to get from the graph, the area under the line.
Is it clear?

You can find the area of column by multiplying its height (y-value of the dataPoint) and width (width of the dataPoint). Please take a look at this JSFiddle, where area of individual columns are calculated when you click on it.