I plotted a curve and needed to seperate it based on weather the y axis exceed a limit. I added a horziontal line at the limit of the y axis, to the plot of the curve. I think it matters that I used pchip and ppval to create my curve ,rather than an actual polynmial. This means when I try finding the intersect with methods such as the intersect funtion, I am returened with a blank array. I am not sure how I can get matlab to simpily just output the intersect efficently , if possible. Note splitting the array of the y values could be hard since it consists of more then 10000 values.