offset in stacked graphs

Hi,

I'm looking for the way to display several line charts (3-4) on one graph. The best way I've found so far, is to use stacked function ('Draw Stacked' in the special functions list), but this one sums the values of the charts.

For example, let's assume i have values 1,2,3,4 for charts: chart1,chart2,chart3,chart4 (for the same timestamp), so it will display chart1 with value 1, chart 2 with value 3 (1+2), chart3 with value 6(1+2+3) and chart4 with value 10(1+2+3+4).

How can I force chart1 to have value 1, chart2 -2, char3 - 3 and chart4 - 4?
I tried to use offset function, but looks like it accepts only constants.

thanks in advance.