title: Display figures and names with tooltips and mark_text with Altair
body: <p>Here are three issues I have with tooltips and labels that I want to display on my Altair graph. All the issues are more or less linked. </p>
  
  <ol>
  <li>First, I would like to modify the name of the information I display with the tooltip:</li>
  </ol>
  
  <p><em>Year</em> instead of <em>properties.annee</em></p>
  
  <p><em>Region</em> instead of <em>properties.region</em></p>
  
  <p><em>Bioenergy</em> instead of <em>properties.bioenerie...</em></p>
  
  <ol start="2">
  <li><p>Second, I would like to round the values displayed in the tooltip.
  "11.2" instead of "11.1687087653"
  The code I wrote does what I want for the labels I put in the regions but it is not working for the tooltip.</p></li>
  <li><p>Third, I would like to display the unit in the labels and in the tooltip but I don't find  the correct syntax in the documentation.</p></li>
  </ol>
  
  <p>Below is my code.</p>
  
  <p>Thanks in advance for yous answers.</p>
  
  <p>Bertrand</p>
  
  <p><a href="https://i.stack.imgur.com/uOggn.png" rel="nofollow noreferrer">Current result of my code</a></p>
  
  <pre><code>    def gen_map(data: gpd.geodataframe.GeoDataFrame, title: str, abs_values: bool):    
          data_json = json.loads(data.to_json())
          choro_data = alt.Data(values=data_json['features'])
  
          # Absolute values or relative values
          if abs_values:
              column = data.columns[0]
              units = 'MW'
              form = '.0f'
          else:
              column = data.columns[1]
              units = '%'
              form = '.1f'
  
          # Base layer
          layer = alt.Chart(choro_data, title=title).mark_geoshape(
              stroke='white',
              strokeWidth=1
          ).encode(
              alt.Color(f'properties.{column}:Q',
                     type='quantitative',
                     title = f'Installed Capacity in {units}'),
          tooltip=[f'properties.annee:Q', 
                   f'properties.region:O', 
                   f'properties.{column}:Q', 
                   alt.Text(f'properties.{column}:Q', format=form)]
           ).transform_lookup(
              lookup='region',
              from_=alt.LookupData(choro_data, 'region')
          ).properties(
              width=600,
              height=500
          )
  
          # Label layer
          labels = alt.Chart(choro_data).mark_text(baseline='top'
                                               ).properties(
              width=600,
              height=500
          ).encode(
              longitude='properties.centroid_lon:Q',
              latitude='properties.centroid_lat:Q',
              text=alt.Text(f'properties.{column}:Q', format=form),
              size=alt.value(14),
              opacity=alt.value(1)        
          )
  
          return layer + labels
  
      gen_map(bioenergies_2019, 'Bioenergy in France in 2019', False)
  </code></pre>