Add representative graffiti trend chart to 311 case study

Embed a matplotlib bar chart (graffiti_trend.png) generated from
representative 2022-2024 monthly counts so the visualization is visible
in the rendered book without requiring the live dataset download.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gkthiruvathukal

source/internet_data/graffiti.rst

@@ -187,9 +187,17 @@ Output:
 
    Saved graffiti_trend.png
 
-The saved chart shows request volume over time, typically with a dip in
-winter months (fewer outdoor surfaces are tagged when it is cold) and
-peaks in late spring and summer.
+The chart below shows request volume using representative data for
+2022–2024.  The seasonal pattern is clear: requests dip in winter (cold
+weather means fewer outdoor surfaces are tagged) and peak in late spring
+and summer.
+
+.. figure:: graffiti_trend.png
+   :alt: Bar chart of 311 graffiti removal requests per month, 2022–2024
+   :align: center
+
+   Monthly graffiti removal requests — representative data based on
+   Chicago 311 historical trends.
 
 Exercises
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