build_ft: A simple frequency trail plot for the waterfall function
In AndreyAkinshin/ggwaterfall: Functions for Drawing Waterfall Plots

You can find the definition of frequency trails in Brendan Gregg's Blog

1 2	build_ft(x, x.lims, fill, col = "black", bw = "SJ", show.underline = T, alpha = 0.8, label.expand = 0)

`x`	The data to be displayed.
`x.lims`	The range of values which should be displayed.
`fill`	The color which will be used for the inside plot area.
`col`	The color which will be used for the plot border.
`bw`	The smoothing bandwidth to be used. The value is passed to `density`. The default value is `bw.SJ`.
`show.underline`	When `TRUE`, a horiznotal line will be drawn under the plot. The default value is `TRUE`.
`alpha`	The alpha level of individual plots. The default value is `0.8`.
`label.expand`	The relative expand value for the right side. The default value is `0.0`.