This function mutates :VegaVisualization, adding/modifying a chart title.
In Vega, there are only concepts of “marks”. What this implies is, you can continually add additional marks as desired, such as for a subtitle.
In Vega.jl, to add a subtitle (or multiple!), just change the y offset value. The larger the value (i.e. less negative until you cross over 0), the further DOWN the chart the title will be located. Note that there is nothing that keeps the titles from overlapping the chart; if you find the text to be in a sub-optimal location, just keep modifying the y parameter until you get the desired outcome.