diff --git a/util/plot_shmoo.py b/util/plot_shmoo.py
index cdcc0f8..b5d8711 100755
--- a/util/plot_shmoo.py
+++ b/util/plot_shmoo.py
@@ -92,6 +92,13 @@ def generate_data(
 
 
 def main(out_file: str, *genargs) -> int:
+    # CLI invocations cannot pass real `None`; the documented usage in this
+    # file's header uses positional placeholders for the optional measurement
+    # arguments, which then arrive here as the literal string "None". Convert
+    # those (and empty strings) to actual `None` so the downstream
+    # `is not None` checks behave as intended.
+    genargs = tuple(None if g in ('None', '') else g for g in genargs)
+
     # Generate data to be plotted
     data = generate_data(*genargs)
 
@@ -99,13 +106,13 @@ def main(out_file: str, *genargs) -> int:
     bar_cmap = mpl.colormaps['viridis']
     bar_cmap.set_under('white')
     bar_show = True
-    if genargs[6] is not None:
+    if len(genargs) > 6 and genargs[6] is not None:
         bar_legend = 'En. Eff. (MFLOP/s/W)'
         bar_data = data['effs_mflop_per_s_per_w']
-    elif genargs[4] is not None:
+    elif len(genargs) > 4 and genargs[4] is not None:
         bar_legend = 'Energy (mJ)'
         bar_data = data['es_mj']
-    elif genargs[1] is not None:
+    elif len(genargs) > 1 and genargs[1] is not None:
         bar_legend = 'Power (mW)'
         bar_data = data['ps_mw']
     else:
@@ -119,22 +126,31 @@ def main(out_file: str, *genargs) -> int:
     fig = plt.figure(figsize=(3.3*scale, 2.0*scale))
     ax = plt.subplot(111)
 
-    # Style axes
-    xdata = data['vs_v']
-    ydata = data['fs_mhz']
-    xticks = xdata[::5]
-    yticks = ydata[::6]
+    # Style axes. The plotted axes are frequency on X (matching xlabel) and
+    # voltage on Y (matching ylabel); name the local variables accordingly so
+    # the tick wiring below cannot drift from the pcolormesh argument order.
+    freq_mhz = data['fs_mhz']
+    volt_v   = data['vs_v']
+    freq_ticks = freq_mhz[::3]
+    volt_ticks = volt_v[::5]
     ax.set_xlabel('Frequency (MHz)', fontsize=10)
     ax.set_ylabel('Core Voltage (V)', fontsize=10)
-    plt.xticks(xticks, rotation=0)
-    ax.set_yticks(yticks)
-    ax.set_xticklabels(xticks)
-    ax.set_yticklabels(yticks)
-    ax.xaxis.set_major_formatter(lambda val: f'{val:.0f}')
-    ax.yaxis.set_major_formatter(lambda val: f'{val:1.2f}')
-
-    # Plot the desired data and save
-    c = ax.pcolormesh(ydata, xdata, bar_data, cmap=bar_cmap, edgecolor='silver', linewidth=0.0)
+    ax.set_xticks(freq_ticks)
+    ax.set_yticks(volt_ticks)
+    ax.set_xticklabels([f'{v:.0f}' for v in freq_ticks])
+    ax.set_yticklabels([f'{v:.2f}' for v in volt_ticks])
+    # Matplotlib >= 3.5 calls the formatter as ``fmt(value, pos)``; the
+    # original 1-arg lambdas raised TypeError during draw, preventing the
+    # plot from ever being saved.
+    ax.xaxis.set_major_formatter(lambda val, _pos: f'{val:.0f}')
+    ax.yaxis.set_major_formatter(lambda val, _pos: f'{val:1.2f}')
+
+    # Plot the desired data and save. pcolormesh(X, Y, C): X = frequency,
+    # Y = voltage. `bar_data` has shape (len(volt_v), len(freq_mhz)) so
+    # `bar_data[volt_idx][freq_idx]` lands at (freq_mhz[freq_idx],
+    # volt_v[volt_idx]).
+    c = ax.pcolormesh(freq_mhz, volt_v, bar_data, cmap=bar_cmap,
+                      edgecolor='silver', linewidth=0.0)
     if bar_show:
         cbar = fig.colorbar(c, ax=ax)
         cbar.set_label(bar_legend, fontsize=10)