Difference between trend line and graph?

Question

I have made a graph that is growing steeper at first, and then grows linear. I have added a trendline to the linear part of the graph, and now I have to find out on which x-value the difference between the trendline and the graph is exactly 18. Can anyone help me?

Answer 1

Unless your graphed line is created from an equation then there is no exact answer. It would depend on how you are interpolating between the points. Assuming that you are using linear interpolation between your points then this is not too bad. You need to determine between which 2 points you are going to find your answer. To do this you need to get the equation for the trend line which you can do by right clicking the trend line and selecting to view the equation. Now just plug in your known x values to that eqations to get the corresponging y values. Subtract these y values from your known y values to find the 2 points where the difference is close to 18 (one side above and the other below). Now it is just a matter of doing a linear calculation to determine the point where the difference is 18.

Answer 2

If you select the trendline and then click the Format Selection button (or press Ctrl+1) you will be able to set the trendline options to display the formula on the chart.  Once you have the formula you can return to your data sheet and create a new column to calculate the trendline value for each x value, you can then compare these values with the values you are plotting to find a point where the difference is 18 (or as near as).