Gaussian Process Ribbon Style Uncertainty Area

I have a function with some upper and lower bounds which express some uncertainty. The style of plot I would like to achieve looks like this

http://www.gaussianprocess.org/gpml/code/matlab/doc/f3.gif
or
http://gpytest2.readthedocs.io/en/latest/_images/tuto_GP_regression_m2.png

The closest I have been able to get to this using the following code from proto-repl-charts format

```
(proto-repl-charts.charts/custom-chart
"Custom"
{:data {:columns
[(cons "x" grid)
(cons "gp" (map (comp probit F) grid))
(cons "gp+" (map (fn [x] (probit (+ (F x) 0.5))) grid))
(cons "gp-" (map (fn [x] (probit (- (F x) 0.5))) grid))]
:types {:gp "spline" :gp+ "area" :gp- "area"}
:colors {:gp- "grey" :gp+ "grey"}
:x "x"}})
```

resulting in


This is already pretty good, but ideally I do not want the area below the lower curve to be grey, I want it to be white.

Does anyone have a better way of coding this using what is currently available? I searched the docs for "ribbon" but couldn't find anything. This would be a really great feature to add.

Start with the proto-repl-charts custom-chart configuration and its area rendering options, then compare the referenced Gaussian-process ribbon examples with the current result. Done means the uncertainty band is shown between the upper and lower curves while the area below the lower curve remains white, using currently available functionality or a documented feature addition.

Written by the indexing model from the issue text.