2 min read

Dynamic graphical object in grid, part 2

I have introduced how to define a dynamic grob by setting its makeContext() method. In this post, I will demonstrate two other ways, which are either identical to the previous method, or very similar to it.

Again, the plotting task: drawing two circles at (1, 0) and (-1, 0) with both radius of 1, and we want the two circles fill the plotting region as much as possible no matter how the graphical device is resized.

theta = seq(0, 2*pi, length = 50)

x1 = cos(theta) + 1
y1 = sin(theta)

x2 = cos(theta) - 1
y2 = sin(theta)

In the first method, we use the grid.delay() function, which accepts an expression and it will be evaluated when the image is redrawn (e.g. by resizing the graphical device).

This method is idential to the use of makeContext()/makeContent() hook functions. grid.delay()/delayGrob() generates a grob in the delayGrob class, and the corresponding S3 method grid:::makeContent.delayedgrob() evaluates the expression.


vp = viewport(xscale = c(-2, 2), yscale = c(-1, 1))
x = grobTree(linesGrob(x1, y1, default.units = "native"), 
             linesGrob(x2, y2, default.units = "native"),
             vp = vp)

    vp_width = convertWidth(x$vp$width, "in", valueOnly = TRUE)
    vp_height = convertHeight(x$vp$height, "in", valueOnly = TRUE)

    if(vp_width > 2*vp_height) {
        x$vp$width = unit(2*vp_height, "in")
        x$vp$height = unit(vp_height, "in")
    } else {
        x$vp$width = unit(vp_width, "in")
        x$vp$height = unit(vp_width/2, "in")
}, list())

Besides makeContext()/makeContent() that define what to do when drawing the graphical object, another set of hook functions (the old style) preDrawDetails()/drawDetails()/postDrawDetails() can also be used. Similar to grid.delay(), grid.record() is a helper function which internally calls the three hook functions. The first argument for grid.record() is also an expression. The difference is, in the expression for grid.delay(), it must return a grob object, while in grid.record() you can put grid.*() plotting functions there.

    vp = current.viewport()
    vp_width = convertWidth(vp$width, "in", valueOnly = TRUE)
    vp_height = convertHeight(vp$height, "in", valueOnly = TRUE)

    if(vp_width > 2*vp_height) {
        w = unit(2*vp_height, "in")
        h = unit(vp_height, "in")
    } else {
        w = unit(vp_width, "in")
        h = unit(vp_width/2, "in")
    pushViewport(viewport(xscale = c(-2, 2), yscale = c(-1, 1), width = w, height = h))
    grid.lines(x1, y1, default.units = "native")
    grid.lines(x2, y2, default.units = "native")
}, list())