Cumulative Total % (Purchase vs Budget)

Trying to figure out a way to calculate the total % of purchases vs the budget. 

We are supposed to maintain spending at the rate in which we have a budget for the year. Budget is $12,000 so we spent about $1000 a month or 8.3%. 

On a line chart I have the X-axis as the months (July - June for fiscal year) and the Y axis is the spend rate in which we should go by about 8.3% per month). So July should be 8.3%, Aug should be 16.6%, sept 24.9%, etc. 

If we buy $1000 in July, $800 in Aug and $900 in Jan, that is 22.5% and should reflect that rate, but it is only showing the 7.5% for that actual month. 

I would like the budget % line to show linear from 0-100% on the chart and then the spent as a cumulative (8.3% in July, about 15% in Aug and 24.9% in Sept.