Assume you have a set of X data points {1.5, 3, 4, 5.5, 7}. These are unequally spaced. You get a start and end X value from your cursors on the graph. Start value is 2.5 and end value is 4.5 from the cursors. You now need to find what index in your points to start with. 2.5 is between 1.5 and 3, indices 0 and 1 in your X array. Similarly, 4.5 is between 4 and 5.5, indices 2 and 3 in your array. You need a function to determine the fractional index of a value given an array of values. Our first point, 2.5, has fractional index 0.67 since it is two thirds of the way between the index 0 and index 1 points. Similarly, the end point has fractional index 2.67 since it is two thirds of the way between indices 2 and 3. The threshold 1D array function gives you this fractional index. Your full X array is the array input. The X value from your cursors is the threshold input. The output will be the fractional index. To use this, you need to convert to any integer. To do this, you need to decide whether you want to round the fractional index up, down, or to the nearest integer. You can use the three rounding primitives in the Numeric palette to do any of these. Once you have your indices, use the array subset primitive from the arrays palette to get the pieces of the X and Y arrays (two subset operations) for integration. Then use the integration routine I mentioned above.
