Convenience functionssemilogx,semilogy, andloglog#

Since plotting data on semi-logarithmic or double-logarithmic scales is very common,the functionssemilogx,semilogy, andloglog are shortcutsfor setting the scale and plotting data; e.g.ax.semilogx(x,y) is equivalent toax.set_xscale('log');ax.plot(x,y).

importmatplotlib.pyplotaspltimportnumpyasnpfig,(ax1,ax2,ax3)=plt.subplots(1,3,layout='constrained',figsize=(7,7/3))# log x axist=np.arange(0.01,10.0,0.01)ax1.semilogx(t,np.sin(2*np.pi*t))ax1.set(title='semilogx')ax1.grid()ax1.grid(which="minor",color="0.9")# log y axisx=np.arange(4)ax2.semilogy(4*x,10**x,'o--')ax2.set(title='semilogy')ax2.grid()ax2.grid(which="minor",color="0.9")# log x and y axisx=np.array([1,10,100,1000])ax3.loglog(x,5*x,'o--')ax3.set(title='loglog')ax3.grid()ax3.grid(which="minor",color="0.9")

Logarithms with other bases#

By default, the log scale is to the base 10. One can change this via thebaseparameter.

fig,ax=plt.subplots()ax.bar(["L1 cache","L2 cache","L3 cache","RAM","SSD"],[32,1_000,32_000,16_000_000,512_000_000])ax.set_yscale('log',base=2)ax.set_yticks([1,2**10,2**20,2**30],labels=['kB','MB','GB','TB'])ax.set_title("Typical memory sizes")ax.yaxis.grid()

Dealing with negative values#

Non-positive values cannot be displayed on a log scale. The scale has two optionsto handle these. Either mask the values so that they are ignored, or clip themto a small positive value. Which one is more suited depends on the type of thedata and the visualization.

The following example contains errorbars going negative. If we mask these values,the bar vanishes, which is not desirable. In contrast, clipping makes the valuesmall positive (but well below the used scale) so that the error bar is drawnto the edge of the Axes.

x=np.linspace(0.0,2.0,10)y=10**xyerr=1.75+0.75*yfig,(ax1,ax2)=plt.subplots(1,2,layout="constrained",figsize=(6,3))fig.suptitle("errorbars going negative")ax1.set_yscale("log",nonpositive='mask')ax1.set_title('nonpositive="mask"')ax1.errorbar(x,y,yerr=yerr,fmt='o',capsize=5)ax2.set_yscale("log",nonpositive='clip')ax2.set_title('nonpositive="clip"')ax2.errorbar(x,y,yerr=yerr,fmt='o',capsize=5)plt.show()

Total running time of the script: (0 minutes 2.372 seconds)

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