               yLabel.append "tspan"
                 .classed unit: true
                 .text "[K]"
+
+          svg.append "g"
+            .classed label: true, y1: true
+            .append "text"
+            .attr "text-anchor": "middle", dy: ".71em", y: -10
+            .call (yLabel) ->
+              yLabel.append "tspan"
+                .text "Relative Pressure, Density "
+              yLabel.append "tspan"
+                .classed unit: true
+                .text "[bar, rho]"
 
       plots = svg.select ".plots"
       solutions = plots.select ".solutions"


              xLabel.append "tspan"
                .classed unit: true
                .text " [km]"

          svg.append "g"
            .classed label: true, y: true
            .append "text"
            .attr "text-anchor": "middle", dy: ".71em"
            .call (yLabel) ->

      el_y0Axis = svg.select ".axis.y0"
      el_y1Axis = svg.select ".axis.y1"
      xLabel = svg.select ".label.x text"
      y0Label = svg.select ".label.y0 text"
      y1Label = svg.select ".label.y1 text"

      # listen for window resize
      # it is still the job of the owner of `selection` to update)
