Fix error when trying to plot labels in a gap in the data

labellines/line_label.py

@@ -165,6 +165,8 @@ def _update_anchors(self):
         mask = np.isfinite(ydata)
         if mask.sum() == 0:
             raise ValueError(f"The line {self._line} only contains nan!")
+        xdata = xdata[mask]
+        ydata = ydata[mask]
 
         # Find the first line segment surrounding x
         for i, (xa, xb) in enumerate(zip(xdata[:-1], xdata[1:])):

labellines/test.py

@@ -243,6 +243,20 @@ def test_nan_failure():
         labelLine(line, 0.5)
 
 
+@pytest.mark.mpl_image_compare
+def test_nan_gaps(setup_mpl):
+    x = np.linspace(0, 10, 100)
+    x[10:30] = np.nan
+
+    for i in range(10):
+        y = np.sin(x + i)
+        plt.plot(x, y, label=f"y={i}")
+
+    labelLines(plt.gca().get_lines())
+
+    return plt.gcf()
+
+
 @pytest.mark.mpl_image_compare
 def test_label_range(setup_mpl):
     x = np.linspace(0, 1)