I´ve created just for the fun two routines to compute and to draw Lagrange Interpolating Polynomial for 3PTS and 4PTS by using fx-cg50 calc. The txt files are included too. No code optimization :-)
The inputs are given coordinates from X0, Y0 to X2, Y2, resp. X3, Y3.
The output are coefficients, graph and plotted given points.
After drawing the program is asking for X, point, in which you can want to see interpolated value Y(X). This step can be skipped.