semilogy — Plot data with a logarithmic y-axis for exponential trends, decay curves, and MATLAB semilogy workflows.
semilogy uses the standard RunMat line-plot path and then marks the active y-axis as logarithmic. It is a natural fit for exponential decay, growth curves, and magnitude data that spans orders of magnitude along Y while preserving MATLAB semilogy call forms.
How semilogy works in RunMat
- The returned value is a numeric line handle.
- Only the y-axis is switched to log scale; the x-axis remains linear.
- Y log-mode state is subplot-local and does not leak into other subplot axes.
- Positive y-values are the meaningful domain for a logarithmic y-axis.
Examples
Plot an exponential decay on a logarithmic y-axis
t = linspace(0, 10, 200);
semilogy(t, exp(-t));Compare multiple decays on the same axes
t = linspace(0, 10, 200);
h1 = semilogy(t, exp(-t));
set(h1, 'DisplayName', 'exp(-t)');
hold on;
h2 = semilogy(t, exp(-0.3*t));
set(h2, 'DisplayName', 'exp(-0.3t)');
legend;Inspect the y-axis scale after plotting
t = 0:0.1:4;
semilogy(t, exp(t));
ax = gca;
get(ax, 'YScale')Expected output:
ans =
'log'FAQ
When is semilogy the right choice over a linear plot?
Whenever the interesting structure lives in multiplicative changes along Y — exponential decay, growth rates, signal attenuation. On a log y-axis, exp(-t) becomes a straight line, which makes decay constants easy to read directly from the slope.
How do I visualize exponential decay and read the time constant?
Plot with semilogy and the decay appears as a straight line. The slope of that line is the decay rate. For exp(-t/tau), the signal drops by a factor of e every tau units along the x-axis.
t = linspace(0, 10, 200);
semilogy(t, exp(-t/3)); % tau = 3
grid on;What about zero or negative y-values on a log y-axis?
They're excluded from rendering since log of zero or negative numbers is undefined. The data handle still stores them, but only positive y-values produce visible points. If your data crosses zero, consider splitting into positive and negative series or using a linear axis.
Related functions to explore
These functions work well alongside semilogy. Each page has runnable examples you can try in the browser.
More plotting resources
Open-source implementation
Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how semilogy works, line by line, in Rust.
- View semilogy.rs on GitHub
- Learn how the runtime works
- Found a bug? Open an issue with a minimal reproduction.
About RunMat
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