Fix vertical scaling of Eisenstein diagrams for p-adic families

lmfdb/local_fields/family.py

@@ -177,7 +177,16 @@ def picture(self):
         ticklook, ticks, slopeset, meanset, rectangles = self.spread_ticks
         hscale = self.e / 18
         mindiff = min((ticklook[b]-ticklook[a]) for (a,b) in rectangles)
-        aspect = min(8, max(0.6 * (self.e + 3*hscale) / (1 + maxslope), self.e/(32*mindiff)))
+        # The image is displayed with a fixed width, so its rendered height is
+        # proportional to aspect * maxslope / xwidth.  The first term gives a
+        # pleasant default shape; the second term increases the height when
+        # needed to keep the closest pair of y-axis labels legibly separated.
+        xwidth = self.e + 3*hscale
+        aspect = max(0.6 * xwidth / (1 + maxslope), self.e/(32*mindiff))
+        # Clamp the height:width ratio of the image to [1/4, 5/4].  A fixed cap
+        # on the aspect itself (which is measured in data coordinates) would
+        # vertically compress families with small slopes and large e (#6828).
+        aspect = min(max(aspect, xwidth/(4*maxslope)), 5*xwidth/(4*maxslope))
 
         # Print the bands
         for (a,b), cnt in rectangles.items():

lmfdb/local_fields/test_localfields.py

@@ -1,3 +1,8 @@
+import re
+import struct
+from base64 import b64decode
+from urllib.parse import unquote
+
 from lmfdb.tests import LmfdbTest
 
 
@@ -60,6 +65,25 @@ def test_search_download(self):
     mapput(~out, "field", field);
     return(out);''' in page
 
+    def _eisenstein_diagram_dims(self, label):
+        # Extract the pixel dimensions of the Eisenstein diagram on a family page
+        page = self.tc.get(f'/padicField/family/{label}').get_data(as_text=True)
+        m = re.search(r'id="diagram_eisenstein"[^>]*>\s*<img src="data:image/png;base64,([^"]*)"', page)
+        assert m is not None
+        png = b64decode(unquote(m.group(1)))
+        assert png[:8] == b'\x89PNG\r\n\x1a\n'
+        return struct.unpack(">II", png[16:24])
+
+    def test_family_diagram_scaling(self):
+        # Families with small slopes and large e used to produce vertically
+        # compressed Eisenstein diagrams with overlapping labels.  Issue #6828.
+        for label in ['2.1.16.16a', '2.1.32.34a']:
+            w, h = self._eisenstein_diagram_dims(label)
+            assert h > 0.3 * w
+        # A typical family should keep a moderate shape
+        w, h = self._eisenstein_diagram_dims('2.1.4.6a')
+        assert 0.25 * w < h < 0.6 * w
+
     def test_families_search_download(self):
         # Absolute families: download should produce a file (not just refresh the page).  Issue #6829.
         r = self.tc.get('/padicField/?Submit=sage&download=1&query=%7B%27n0%27%3A+1%2C+%27p%27%3A+2%2C+%27n%27%3A+2%7D&p=2&n=2&search_type=Families')