Retrieve max count from Histogram

The idea is to create a histogram plot and draw a vertical line of the mean of the data over the histogram.

`a=runif(100);
p <- plot_ly(x=a,type="histogram")`

then add a trace

`p %>% add_trace(x=c(mean(a),mean(a)),y=c(0,maxCount),mode="lines",name="Mean")`

The point is that maxCount is available only when the histogram chart is created

Any suggestion?
