[SVG] Noise hack and font fix (#3076)

 - Specify the font. This should not be a change for viewing in Jupyter notebook but fixes opening the svg data in e.g. inkscape where the default font is different and the boxes aren't the right size
 - Don't display virtual tags (for noise) xref #2905

cirq/contrib/svg/svg.py

@@ -6,12 +6,25 @@
     import cirq
 
 QBLUE = '#1967d2'
+FONT = "Arial"
+
+
+def fixup_text(text: str):
+    if '\n' in text:
+        return '?'
+    if '[<virtual>]' in text:
+        # https://github.com/quantumlib/Cirq/issues/2905
+        # TODO: escape angle brackets when you actually want to display tags
+        return text.replace('[<virtual>]', '')  # coverage: ignore
+    if '[cirq.VirtualTag()]' in text:
+        # https://github.com/quantumlib/Cirq/issues/2905
+        return text.replace('[cirq.VirtualTag()]', '')
+    return text
 
 
 def _get_text_width(t: str) -> float:
-    if '\n' in t:
-        return _get_text_width('?')
-    tp = matplotlib.textpath.TextPath((0, 0), t, size=14, prop='Arial')
+    t = fixup_text(t)
+    tp = matplotlib.textpath.TextPath((0, 0), t, size=14, prop=FONT)
     bb = tp.get_extents()
     return bb.width + 10
 
@@ -30,7 +43,8 @@ def _rect(x: float,
 def _text(x: float, y: float, text: str, fontsize: int = 14):
     """Draw SVG <text> text."""
     return f'<text x="{x}" y="{y}" dominant-baseline="middle" ' \
-           f'text-anchor="middle" font-size="{fontsize}px">{text}</text>'
+           f'text-anchor="middle" font-size="{fontsize}px" ' \
+           f'font-family="{FONT}">{text}</text>'
 
 
 def _fit_horizontal(tdd: 'cirq.TextDiagramDrawer',
@@ -208,13 +222,10 @@ def tdd_to_svg(
         if v.text == '×':
             t += _text(x, y + 3, '×', fontsize=40)
             continue
-        if '\n' in v.text:
-            t += _rect(boxx, boxy, boxwidth, boxheight)
-            t += _text(x, y, '?', fontsize=18)
-            continue
 
+        v_text = fixup_text(v.text)
         t += _rect(boxx, boxy, boxwidth, boxheight)
-        t += _text(x, y, v.text, fontsize=14 if len(v.text) > 1 else 18)
+        t += _text(x, y, v_text, fontsize=14 if len(v_text) > 1 else 18)
 
     t += '</svg>'
     return t

cirq/contrib/svg/svg_test.py

@@ -22,6 +22,15 @@ def test_svg():
     assert '</svg>' in svg_text
 
 
+def test_svg_noise():
+    noise_model = cirq.ConstantQubitNoiseModel(cirq.DepolarizingChannel(p=1e-3))
+    q = cirq.LineQubit(0)
+    circuit = cirq.Circuit(cirq.X(q))
+    circuit = cirq.Circuit(noise_model.noisy_moments(circuit.moments, [q]))
+    svg = circuit_to_svg(circuit)
+    assert '>D(0.001)</text>' in svg
+
+
 def test_validation():
     with pytest.raises(ValueError):
         circuit_to_svg(cirq.Circuit())