Set minimum ylim for phase/group delay plots

Fixes Make plots have minimum `ylim` #179

docs/examples/fir-filters.ipynb

@@ -290,7 +290,6 @@
    "source": [
     "plt.figure(figsize=(10, 5))\n",
     "sdr.plot.group_delay(fir)\n",
-    "plt.ylim(fir.delay - 2, fir.delay + 2)\n",
     "plt.show()"
    ]
   },
@@ -315,7 +314,6 @@
    "source": [
     "plt.figure(figsize=(10, 5))\n",
     "sdr.plot.group_delay(fir, x_axis=\"log\")\n",
-    "plt.ylim(fir.delay - 2, fir.delay + 2)\n",
     "plt.show()"
    ]
   },
@@ -381,7 +379,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.9"
+   "version": "3.8.10"
   },
   "orig_nbformat": 4
  },

src/sdr/plot/_filter.py

@@ -11,7 +11,7 @@
 
 from .._filter import FIR, IIR
 from .._helper import export
-from ._helper import real_or_complex_plot
+from ._helper import min_ylim, real_or_complex_plot
 from ._rc_params import RC_PARAMS
 from ._units import freq_units, time_units
 
@@ -618,6 +618,8 @@ def phase_delay(
         else:
             plt.plot(f, tau_phi, **kwargs)
 
+        min_ylim(tau_phi, 2 / sample_rate, sample_rate)
+
         plt.grid(True, which="both")
         if "label" in kwargs:
             plt.legend()
@@ -668,8 +670,7 @@ def group_delay(
             h_srrc = sdr.root_raised_cosine(0.5, 10, 10)
             @savefig sdr_plot_group_delay_1.png
             plt.figure(figsize=(8, 4)); \
-            sdr.plot.group_delay(h_srrc); \
-            plt.ylim(48, 52)
+            sdr.plot.group_delay(h_srrc);
 
         See the :ref:`iir-filters` example.
 
@@ -687,8 +688,7 @@ def group_delay(
 
             @savefig sdr_plot_group_delay_3.png
             plt.figure(figsize=(8, 4)); \
-            sdr.plot.group_delay(h_srrc, x_axis="two-sided"); \
-            plt.ylim(48, 52)
+            sdr.plot.group_delay(h_srrc, x_axis="two-sided");
 
         .. ipython:: python
 
@@ -739,6 +739,8 @@ def group_delay(
         else:
             plt.plot(f, tau_g, **kwargs)
 
+        min_ylim(tau_g, 2 / sample_rate, sample_rate)
+
         plt.grid(True, which="both")
         if "label" in kwargs:
             plt.legend()

src/sdr/plot/_helper.py

@@ -33,3 +33,13 @@ def real_or_complex_plot(
 
     if label:
         plt.legend()
+
+
+def min_ylim(y: npt.NDArray, separation: float, sample_rate: float):
+    ymin, ymax = plt.gca().get_ylim()
+    if ymax - ymin < separation:
+        # Find the mean of the signal rounded to the nearest sample
+        mean = int(round(np.mean(y) / sample_rate))
+        ymin = mean - separation / 2
+        ymax = mean + separation / 2
+        plt.gca().set_ylim(ymin, ymax)