Rendering arrows using text() instead of polygon()

[vsimko]

Hi Tarik @tgouhier,

I have a question regarding the arrow() function.
Currently, the function generates a polygon representing the arrow. There are two parameters to control the size of the arrow - width and length.

I think it would be better to print an arrow using the text() function as a character at specified coordinates. This would allow users to use different arrow shapes.

Take a look at my implementation here - arrow2() function:
https://github.com/tgouhier/biwavelet/blob/master/R/phase.plot.R

Notice that my function uses just a single size parameter.

Here are some examples how it might look like when using arrow2 instead of arrow:

[vsimko]

I would suggest to:

• changing arrow.len to arrow.size
• removing arrow.lwd parameter which can always be computed from arrow.size.
• keeping arrow.col
• adding arrow.symbol which would be:
  • "polygon" or NULL or NA for the original polygon-based arrow (default?)
  • single character which would be printed using the text() function

[vsimko]

default could also be a character that we know is safe on all platforms.
I think that intToUtf8(0x279B) ("➛" Drafting Point Rightwards Arrow) might be that character.

[vsimko]

My implementation is half-finished, but I stumbled upon the following problems:

• testthat-based unit tests don't play well with UTF-8 characters. The following error occurs: conversion failure on '⤑' in 'mbcsToSbcs': dot substituted for <a4>. I would therefore suggest to use character code instead of the character itself, e.g. symbol=0x279b unstead of symbol="➛"

• When a plot is rendered and later resized, polygon-based arrows keep their shape exactly as rendered while character-based arrows will be automatically resized to keep their aspect-ratio. Both approaches have their advantages.