build_pile_plot: Build a multi-track pileup plot
In AllenInstitute/lowcat: Low Coverage Accessibility and Transcriptomics

Build a multi-track pileup plot

build_pile_plot(gr_list, ucsc_loc, highlight_loc = NULL,
  padding = c(1e+05, 1e+05), gr_groups = NULL, group_colors = NULL,
  norm = c("PM", "max"), max_val = NULL, window_size = NULL,
  window_mode = c("max", "mean", "median"), target_color = "#B7B7B7",
  highlight_color = "#F9ED32")

`gr_list`	A list of GRanges objects
`ucsc_loc`	A target location, in UCSC format (e.g. "chr1:533,235-552,687)
`highlight_loc`	A location to use for highlights in UCSC format
`padding`	A 2-element numeric vector with upstream and downstream padding around the ucsc_loc to extend the plotting window.
`gr_groups`	A vector indicating which group each GRanges object in gr_list belongs to. If NULL, will group all samples into a single track.
`group_colors`	A named vector, one per group, with colors for each group. Names should match values in gr_groups.
`norm`	Normalization for each group. Currently support is per-million ("PM") and "max".
`max_val`	A maximum value to use for scaling the y-values in each track.
`window_size`	Window bin size to use for region down-sampling. If NULL, will not downsample.
`window_mode`	If using window_size, what value to use for each window. Options are "max","mean", and "median".
`target_color`	The color of the background rectangle to highlight the region in ucsc_loc.
`highlight_color`	The color the background rectangle to highlight the region in highlight_loc.