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## matplotlib-users

 [Matplotlib-users] Need help with direction field plot From: MedEng1 - 2010-06-11 20:58:21 ```I'm relatively new to matplotlib so I need some help. I'm trying to make a direction field plot for visualzing solutions to differential eqs. My first attempt at a simple plot came out all right but I can't figure out why the plotted lines are not the same length. The block of code that does all of the work is... for x in ax1.xaxis.iter_ticks(): #Iterate over x-ticks for y in ax1.yaxis.iter_ticks(): #iterate over y-ticks # Solve for local slope at point to be evaluated (dy/dx) yp = 9.8 - ((np.float(y[1]))/4) #yp = y` as in dy/dx # Plot line based on slope cpx = np.float(x[1]) #cpx = x-coord of line center, convert x-tick value to float cpy = np.float(y[1]) #cpy = y-coord of line center, covnert y-tick value to float a = np.arctan(yp) #a = angle between line and horizontal px = np.cos(a)*(L/2) #px = x offset from line center to end py = np.sin(a)*(L/2) #py = y offset from line center to end tpx = cpx + px #tpx = x-coord of right hand end of line tpy = cpy + py #tpy = y-coord of right hand end of line bpx = cpx - px #bpx = x-coord of left hand end of line bpy = cpy - py #bpy = y-coord of left hand end of line Length = np.sqrt(((tpx-bpx)**2)+((tpy-bpy)**2)) #just a quick check of the line length accoring to the coordinates of the ends ax1.plot([bpx, tpx], [bpy, tpy], color='black', linewidth=0.75) As you can see in the attached figure (see below), the lines seem to plot in the right locations and right angles but the lengths are not constant (not intended - seem to be twice as long near the middle of the chart). Am I missing something obvious? Why are the lines near the center of the chart longer? Thanks in advance for any help provided. -Ryan http://old.nabble.com/file/p28859981/img1.png -- View this message in context: http://old.nabble.com/Need-help-with-direction-field-plot-tp28859981p28859981.html Sent from the matplotlib - users mailing list archive at Nabble.com. ```