Matplotlib. co-ordinates for scatter plot are off

EricGarlic eric.garlic at gmail.com
Tue Nov 26 20:44:16 CET 2013


On Tuesday, November 26, 2013 7:34:31 PM UTC, EricGarlic wrote:
> 
Correction:  I meant Plot not Scatter - corrected below.  Issue is with Plot statement


Hi, 
> 
> 
> 
> I have been following a very good online tutorial for matplotlib: 
> 
> 
> 
> http://www.loria.fr/~rougier/teaching/matplotlib/#introduction
> 
> 
> 
> However, when I try to annotate the point where the cosine/sine graphs cross the plot graph misses the point where it crosses the axes; green and pink colours.
> 
> 
> 
> Anyone know why and/or what I have done wrong?
> 
> 
> 
> Help appreciated
> 
> 
> 
> 
> 
> _______________________CODE BELOW_________________
> 
> 
> 
> #the points
> 
> x = np.linspace(-np.pi, np.pi, 256, endpoint=True)
> 
> C,S=np.cos(x), np.sin(x)
> 
> 
> 
> #axis limits
> 
> xlim(x.min()*1.1, x.max()*1.1)
> 
> ylim(C.min()*1.1,C.max()*1.1)
> 
> 
> 
> 
> 
> #spines, there are four (top, bottom, left and right).
> 
> #spines, remove top and right, and move left and bottom
> 
> #gca returns current axis instance.  Used to control axis properties using either set or "Axes" methods
> 
> #I assume "gca" stands for Get Current Axis, like "gcf" stands for Get Current Figure
> 
> ax=gca()
> 
> #spines
> 
> ax.spines['right'].set_color('none')
> 
> ax.spines['top'].set_color('none')
> 
> #ticks
> 
> ax.xaxis.set_ticks_position('bottom')
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> ax.yaxis.set_ticks_position('left')
> 
> #reposition
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> ax.spines['left'].set_position(('data',0))
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> ax.spines['bottom'].set_position(('data',0))
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> 
> 
> 
> 
> #ticks ticks as tuples and labels as LaTex tuples.
> 
> #Note: if these next two lines of code go before the spine changes the labelling is lost
> 
> xticks([-np.pi,-np.pi/2,0,np.pi/2,np.pi],[r'$-\pi$',r'$-\pi/2$',r'$0$',r'$+\pi/2$',r'$+\pi$'])
> 
> yticks([-1,0,1],[r'$-1$',r'0',r'$+1$'])
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> 
> 
> 
> 
> #plotting
> 
> plot(x,C,color="blue", linewidth = 2.5, linestyle="dotted",label = 'Sine')
> 
> plot(x,S, color="red", linewidth="2.5",linestyle="-.", label='Cosine')
> 
> 
> 
> #legend. Note remember to add label to plot statement e.g. 'Sine' as the legend requires it
> 
> #NOTE:  legend has to go after plot statement in order to get the graph labels
> 
> legend(loc='upper left')
> 
> 
> 
> #Annotation
> 
> #the green point.  NOTE: use the scatter method
> 
> t=pi/4
> 
> scatter([t,],[np.cos(t),],color='green',linewidth=10)
> 
> #draw the lines with plot
> 
> plot([t,cos(t)],[t,0],color='green')
> 
> plot([0,cos(t)],[t,cos(t)],color='pink')
> 
> 
> 
> scatter([t,],[0,],color='black')
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> scatter([0,],[cos(t),],color='black')
> 
> 
> 
> show()




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