feat: update mpl-size documentation and examples; add DPI explanation

Author: c0rychu

docs/mpl/mpl-size-lg.png

@@ -1,3 +1,5 @@
+## Decreasing Figure Size to Increase Font Size
+
 The first and the most important trick when plotting with Matplotlib is to set the figure size appropriately.
 If you want to increase the font size and line width everywhere in the figure, the easiest way is to **decrease the figure size**!
 
@@ -11,4 +13,22 @@ If you want to increase the font size and line width everywhere in the figure, t
 ```
 
 ![small size](mpl-size-sm.png) 
-![large size](mpl-size-lg.png)
+![large size](mpl-size-lg.png)
+
+
+## DPI
+
+The native figure size unit in Matplotlib is inches as documented [here](https://matplotlib.org/stable/gallery/subplots_axes_and_figures/figure_size_units.html).
+When we intentionally set a small figure size (e.g., 4x2 inches), we can increase the DPI (dots per inch) to get a larger (higher resolution / size in pixels) image without changing the relative sizes of text elements.
+
+The DPI can be set when creating the figure using the `dpi` argument:
+
+```python
+import matplotlib.pyplot as plt
+fig, ax = plt.subplots(figsize=(4, 2), dpi=600)
+```
+or specified when saving the figure using the `dpi` argument of `savefig`:
+
+```python
+fig.savefig("figure.png", dpi=600)
+```

docs/mpl/mpl-size-sm.png

@@ -3,23 +3,21 @@
 
 plt.style.use("physics_plot.pp_base")
 
+x = np.linspace(0, 10, 1000)
+y = np.sin(5 * x)
+
 fig, ax = plt.subplots(figsize=(4, 2), constrained_layout=True)
-x = np.linspace(0, 10, 100)
-y = np.sin(x)
-ax.plot(x, y, label="Sine Wave")
+ax.plot(x, y, label=r"$y = \sin(5x)$")
 ax.set_ylabel(r"$y(x)$")
 ax.set_xlabel(r"$x$")
 ax.legend(loc="upper right")
-fig.suptitle(r"\texttt{figsize=(4, 2)}")
+fig.suptitle(r"Small Figure Size (4x2 inches), i.e., \texttt{figsize=(4, 2)}")
 fig.savefig("mpl-size-sm.png")
 
-
-fig, ax = plt.subplots(figsize=(10, 5), constrained_layout=True)
-x = np.linspace(0, 10, 100)
-y = np.sin(x)
-ax.plot(x, y, label="Sine Wave")
+fig, ax = plt.subplots(figsize=(8, 4), constrained_layout=True)
+ax.plot(x, y, label=r"$y = \sin(5x)$")
 ax.set_ylabel(r"$y(x)$")
 ax.set_xlabel(r"$x$")
 ax.legend(loc="upper right")
-fig.suptitle(r"\texttt{figsize=(10, 5)}")
+fig.suptitle(r"Large Figure Size (8x4 inches), i.e., \texttt{figsize=(8, 4)}")
 fig.savefig("mpl-size-lg.png")