Fix. Account for line style = "none". Closes #91.

[trekonom]

Hi David and Eli,

just in case you find the time. This PR proposes a fix for - the not very well reported - issue #91 .

Using the current CRAN and dev version setting the line style in a ms_linechart to "none" via chart_data_line_style does not work. Instead we get a solid line with the default settings for color, ....:

library(mschart)
library(officer)

dat <- data.frame(
  color = c(rep("green", 3), rep("unclear", 3), rep("gray", 3)),
  musician = c(rep(c("Robert Wyatt", "John Zorn", "Damon Albarn"), 3)),
  count = c(120, 101, 131, 200, 154, 187, 122, 197, 159),
  stringsAsFactors = FALSE
)

lty <- c(green = "none", unclear = "dashed", gray = "none")
lty <- lty[order(names(lty))]

chart <- ms_linechart(data = dat, x = "musician", y = "count", group = "color")
chart <- chart_data_line_style(chart, values = lty)

file <- tempfile(fileext = ".pptx")
doc <- read_pptx()
doc <- add_slide(doc, layout = "Title and Content", master = "Office Theme")
doc <- ph_with(doc, value = chart, location = ph_location_fullsize())
print(doc, target = file)

# doconv::to_miniature(file, fileout = "issue-91.png")

The issue is that currently an empty string is "added" for lines with a "none" style:

mschart/R/utils.R

Lines 51 to 53 in b69fbf6

	if( is_transparent || x$width < 0.001 || x$style %in% "none" ){
	return("")
	}

instead of the required "<c:spPr><a:ln><a:noFill/></a:ln></c:spPr>" as is already done in

mschart/R/to_pml.R

Line 81 in b69fbf6

line_str <- "<c:spPr><a:ln><a:noFill/></a:ln></c:spPr>"

when the style of the linechart is set globally using e.g. chart_settings(x, style = "marker").

The proposed fix adds a none_as_empty argument to ooxml_fp_border so that one can choose between having an empty string (where this is fine) or a <a:ln><a:noFill/></a:ln> for a "none" style.

Using the proposed fix we get the correct result: