For ggplotly: implement lineend = "square" for geom_segment

Example code:

library(ggplot2)
b <- ggplot(mtcars, aes(wt, mpg)) 
df <- data.frame(x1 = c(2.62, 2.62), x2 = c(3.57,2.62), y1 = c(21,21), y2 = c(21, 16))
b + geom_segment(aes(x = x1, y = y1, xend = x2, yend = y2, colour = "segment"), data = df, size = 2, lineend = "square")

library(plotly)
ggplotly()

Here is how the corner looks in ggplots2:

And in ggplotly:

Notice the "notch" in the corner...

首先运行随附的 R 示例，使用 ggplot2 和 plotly，然后比较 ggplot2 与 ggplotly 中方形线端和拐角的渲染效果。issue 中没有指定源文件或测试；当 geom_segment 使用 lineend = "square" 渲染时不再出现所报告的缺口，并且与参考外观一致，即视为完成。