title: Combining rows in a Geopandas Dataframe
body: <p><strong>TLDR: I'm trying to combine rows of a GeoPandas Dataframe into one row where their shapes are combined into one.</strong></p>
  <p>I'm currently working on a little project that requires me to create interactive choropleth plots of Canadian health regions using a few different metrics.</p>
  <p>I had merged two Dataframes, one containing population estimates by year for each health region, and another GeoDataframe containing the geometry for the health regions, when I noticed that the number of rows wasn't the same.</p>
  <p>Upon further inspection, I realized the two datasets I had been using didn't include the exact same health regions. The shape-files I got had a few more health regions than the population data, which had amalgamated a few of them for methodological reasons.</p>
  <p>After noticing the difference, I redid the merge to show me the differences so I could figure out what I need to roll up.</p>
  <pre class="lang-py prettyprint-override"><code>merged_gdf = gdf.merge(df, on='HR_UID') 
  #HR_UID is just the name of the column with the codes for the health regions, since they   
  #have slightly different names in different datasets, it's easier to merge on code.
  print(list(set(df['HEALTH_REGION'])-set(merged_gdf['HEALTH_REGION_y'])),list(set(gdf['HR_UID'])-set(df['HR_UID'].unique())))
  </code></pre>
  <p>Here I was shown the missing health region was ['Mamawetan/Keewatin/Athabasca, Saskatchewan']. The GeoDataframe has those three regions separate, with codes 4711, 4712, 4713, while the population data has them rolled up into one region with code 4714.</p>
  <p>I intend on combining the rows of my GeoDataframe that correspond to the health regions combined in the population data, to combine their polygons.
  I went back to the GeoDataframe to try and combine the three rows corresponding to those regions:</p>
  <pre><code>old_hr=gdf[gdf['HR_UID'].isin({'4711','4712','4713'})]
  </code></pre>
  <pre class="lang-py prettyprint-override"><code>  HR_UID                                      HEALTH_REGION    SHAPE_AREA  \
  31   4711  Mamawetan Churchill River Regional Health Auth...  1.282120e+11   
  32   4712          Keewatin Yatthé Regional Health Authority  1.095536e+11   
  33   4713                         Athabasca Health Authority  5.657720e+10   
  
         SHAPE_LEN                                           geometry  
  31  1.707619e+06  POLYGON ((5602074.666 2364598.029, 5591985.366...  
  32  1.616297e+06  POLYGON ((5212469.723 2642030.691, 5273110.000...  
  33  1.142962e+06  POLYGON ((5248633.914 2767057.263, 5249285.640...  
  
  </code></pre>
  <p>Now I've come to the realization that I'm not exactly sure how to combine polygons in a GeoDataframe. I have tried using <code>dissolve(on='HEALTH_REGION')</code>, although that didn't work. I've spent a while looking around online, but thus far it seems I can't find anyone asking this particular question - perhaps I'm missing something..</p>