The Tempest software’s spline curve fitting is prone to overcompensating interpolations at abrupt changes in slope. I suggest you investigate Stineman curve interpolation which virtually guarantees that curves will pass smoothly through the data points without extraneous “wiggles” in the interpolated curve fits. Refer to an article published in 1980 in Creative Computing. The article fully explains the algorithm. I have never had Stineman interpolation fitting fail to create an excellent curve fit. It’s computationally efficient as well IMHO.
it’s even worse, as they not only interpolate in the y-values of the graph, but also the x, which causes total impossible graphs.
The whole graphing system needs a big overhaul, with easy zoom/pan (two finger/mouse wheel zooming, etc), with way more than the current 32 visible data points (make it 500 for example), with multiple graphs on top of each other (to discover relationships between for example temperature and windspeed), etc etc.