I’m trying to define a `stackedhist`

recipes which takes `bins`

and `array{array}`

and uses groupedbar to display a stacked histogram, the following **function** works:

```
using StatsPlots, StatsBase
function stack_plot(args...)
bins = args[1]
# the weights of bar chart are vertical vector
weights_ary = mapreduce(x->fit(StatsBase.Histogram, x, bins).weights, hcat, args[2])
groupedbar(bins[1:end-1], weights_ary, bar_position = :stack)
end
stack_plot(0:0.1:1, [randn(100), randn(100)])
```

However, the following recipe does not work:

```
using StatsPlots, StatsBase
@userplot StackedHist
@recipe function f(h::StackedHist)
seriestype := :groupedbar
bar_position = :stack
bins = h.args[1]
# the weights of bar chart are vertical vector
weights_ary = mapreduce(x->fit(StatsBase.Histogram, x, bins).weights, hcat, h.args[2])
bins[1:end-1], weights_ary
end
###
ERROR: The backend must not support the series type Val{:groupedbar}, and there isn't a series recipe defined.
```

Am I doing something obvious+stupid thing here?