Draw a line with a label, by default its equation

This is an extension of the panel functions panel.abline and panel.lmline to also draw a label on the line. The default label is the line equation, and optionally the R squared value of its fit to the data points.

panel.ablineq(a = NULL, b = 0,
              h = NULL, v = NULL,
              reg = NULL, coef = NULL,
              pos = if (rotate) 1 else NULL,
              offset = 0.5, adj = NULL,
              at = 0.5, x, y,
              rotate = FALSE, srt = 0,
              label = NULL,
              varNames = alist(y = y, x = x),
              varStyle = "italic",
              fontfamily = "serif",
              digits = 3,
              r.squared = FALSE, sep = ", ", sep.end = "",
              col, col.text, col.line,
              ..., reference = FALSE)

panel.lmlineq(x, y, ...)

Arguments

a, b, h, v, reg, coef

specification of the line. The simplest usage is to give a and b to describe the line y = a + b x. Horizontal or vertical lines can be specified as arguments h or v, respectively. The first argument (a) can also be a model object produced by lm. See panel.abline for more details.

pos, offset

passed on to panel.text. For pos: 1 = below, 2 = left, 3 = above, 4 = right, and the offset (in character widths) is applied.

adj

passed on to panel.text. c(0,0) = above right, c(1,0) = above left, c(0,1) = below right, c(1,1) = below left; offset does not apply when using adj.

fontfamily

passed on to panel.text.

at

position of the equation as a fractional distance along the line. This should be in the range 0 to 1. When a vertical line is drawn, this gives the vertical position of the equation.

x, y

position of the equation in native units. If given, this over-rides at. For panel.lmlineq this is the data, passed on as lm(y ~ x).

rotate, srt

set rotate = TRUE to align the equation with the line. This will over-ride srt, which otherwise gives the rotation angle. Note that the calculated angle depends on the current device size; this will be wrong if you change the device aspect ratio after plotting.

label

the text to draw along with the line. If specified, this will be used instead of an equation.

varNames

names to display for x and/or y. This should be a list like list(y = "Q", x = "X") or, for mathematical symbols, alist(y = (alpha + beta), x = sqrt(x[t])).

varStyle

the name of a plotmath function to wrap around the equation expression, or NULL. E.g. "bolditalic", "displaystyle".

digits

number of decimal places to show for coefficients in equation.

r.squared

the \(R^2\) statistic to display along with the equation of a line. This can be given directly as a number, or TRUE, in which case the function expects a model object (typically lm) and extracts the \(R^2\) statistic from it.

sep, sep.end

The \(R^2\) (r.squared) value is separated from the equation by the string sep, and also sep.end is added to the end. For example: panel.ablineq(lm(y ~ x), r.squared = TRUE, sep = " (", sep.end = ")").

..., col, col.text, col.line

passed on to panel.abline and panel.text. Note that col applies to both text and line; col.text applies to the equation only, and col.line applies to line only.

reference

whether to draw the line in a "reference line" style, like that used for grid lines.

Details

The equation is constructed as an expression using plotmath.

Author

Felix Andrews felix@nfrac.org

Examples

set.seed(0)
xsim <- rnorm(50, mean = 3)
ysim <- (0 + 2 * xsim) * (1 + rnorm(50, sd = 0.3))

## basic use as a panel function
xyplot(ysim ~ xsim, panel = function(x, y, ...) {
  panel.xyplot(x, y, ...)
  panel.ablineq(a = 0, b = 2, adj = c(0,1))
  panel.lmlineq(x, y, adj = c(1,0), lty = 2,
                col.line = "grey", digits = 1)
})


## using layers:
xyplot(ysim^2 ~ xsim) +
  layer(panel.ablineq(lm(y ~ x, subset = x <= 3),
    varNames = alist(y = y^2, x = x[x <= 3]), pos = 4))


## rotated equation (depends on device aspect at plotting time)
xyplot(ysim ~ xsim) +
  layer(panel.ablineq(lm(y ~ x), rotate = TRUE, at = 0.8))


## horizontal and vertical lines
xyplot(ysim ~ xsim) +
  layer(panel.ablineq(v = 3, pos = 4, at = 0.1, lty = 2,
                      label = "3.0 (critical value)")) +
  layer(panel.ablineq(h = mean(ysim), pos = 3, at = 0.15, lty = 2,
                      varNames = alist(y = plain(mean)(y))))


## using layer styles, r.squared
xyplot(ysim ~ xsim) +
  layer(panel.ablineq(lm(y ~ x), r.sq = TRUE,
                      at = 0.4, adj=0:1), style = 1) +
  layer(panel.ablineq(lm(y ~ x + 0), r.sq = TRUE,
                      at = 0.6, adj=0:1), style = 2)


## alternative placement of equations
xyplot(ysim ~ xsim) +
  layer(panel.ablineq(lm(y ~ x), r.sq = TRUE, rot = TRUE,
                      at = 0.8, pos = 3), style = 1) +
  layer(panel.ablineq(lm(y ~ x + 0), r.sq = TRUE, rot = TRUE,
                      at = 0.8, pos = 1), style = 2)


update(trellis.last.object(),
  auto.key = list(text = c("intercept", "no intercept"),
                  points = FALSE, lines = TRUE))