title: How does one control the appearance (e.g. line size, line type, colour) of mqgam plots produced using plot.mgamViz from the "mgcViz" package?
body: <p>I am using quantile regression in R with the <em>qgam</em> package and visualising them using the <em>mgcViz</em> package, but I am struggling to understand how to control the appearance of the plots. The package effectively turns gams (in my case mqgams) into ggplots.</p>
  
  <p>Simple reprex: </p>
  
  <pre><code>egfit &lt;- mqgam(data = iris,
                 Sepal.Length ~ s(Petal.Length),
                 qu = c(0.25,0.5,0.75))
  
  plot.mgamViz(getViz(egfit))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ml3a2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ml3a2.png" alt="enter image description here"></a></p>
  
  <p>I am able to control things that can be added, for example the axis labels and theme of the plot, but I'm struggling to effect things that would normally be addressed in the aes() or geom_x() functions.</p>
  
  <ol>
  <li><p>How would I control the thickness of the line? If this were a normal geom_smooth() or geom_line() I'd simply put size = 1 inside of the geoms, but I cannot see how I'd do so here.</p></li>
  <li><p>How can I control the linetype of these lines? The "id" is continuous and one cannot supply a linetype to a continuous scale. If this were a nomral plot I would convert "id" to a character, but I can't see a way of doing so with the <em>plot.mgamViz</em> function.</p></li>
  <li><p>How can I supply a new colour scale? It seems as though if I provide it with a new colour scale it invents new ID values to put on the legend that don't correlate to the actual "id" values, e.g.</p>
  
  <p>plot.mgamViz(getViz(egfit)) + scale_colour_viridis_c()</p></li>
  </ol>
  
  <p><a href="https://i.stack.imgur.com/p6BP7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/p6BP7.png" alt="enter image description here"></a></p>
  
  <p>I fully expect this to be relatively simple and I'm missing something obvious, and imagine the answer to all three of these subquestions are very similar to one another. Thanks in advance.</p>