R: Add an Axis to a Plot

axis {graphics} | R Documentation

## Add an Axis to a Plot

### Description

Adds an axis to the current plot, allowing the specification of the side, position, labels, and other options.

### Usage

    axis(side, at = NULL, labels = TRUE, tick = TRUE, line = NA,
         pos = NA, outer = FALSE, font = NA, lty = "solid",
         lwd = 1, lwd.ticks = lwd, col = NULL, col.ticks = NULL,
         hadj = NA, padj = NA, gap.axis = NA, ...)

### Arguments

`side` | an integer specifying which side of the plot the axis is to be drawn on. The axis is placed as follows: 1=below, 2=left, 3=above and 4=right.
`at` | the points at which tick-marks are to be drawn. Non-finite (infinite, `NaN` or `NA`) values are omitted. By default (when `NULL`) tickmark locations are computed, see ‘Details’ below.
`labels` | this can either be a logical value specifying whether (numerical) annotations are to be made at the tickmarks, or a character or expression vector of labels to be placed at the tickpoints. (Other objects are coerced by `as.graphicsAnnot`.) If this is not logical, `at` should also be supplied and of the same length. If `labels` is of length zero after coercion, it has the same effect as supplying `TRUE`.
`tick` | a logical value specifying whether tickmarks and an axis line should be drawn.
`line` | the number of lines into the margin at which the axis line will be drawn, if not `NA`.
`pos` | the coordinate at which the axis line is to be drawn: if not `NA` this overrides the value of `line`.
`outer` | a logical value indicating whether the axis should be drawn in the outer plot margin, rather than the standard plot margin.
`font` | font for text. Defaults to `par("font")`.
`lty` | line type for both the axis line and the tick marks.
`lwd, lwd.ticks` | line widths for the axis line and the tick marks. Zero or negative values will suppress the line or ticks.
`col, col.ticks` | colors for the axis line and the tick marks respectively. `col = NULL` means to use `par("fg")`, possibly specified inline, and `col.ticks = NULL` means to use whatever color `col` resolved to.
`hadj` | adjustment (see `par("adj")`) for all labels parallel (‘horizontal’) to the reading direction. If this is not a finite value, the default is used (centring for strings parallel to the axis, justification of the end nearest the axis otherwise).
`padj` | adjustment for each tick label perpendicular to the reading direction. For labels parallel to the axes, `padj = 0` means right or top alignment, and `padj = 1` means left or bottom alignment. This can be a vector given a value for each string, and will be recycled as necessary. If `padj` is not a finite value (the default), the value of `par("las")` determines the adjustment. For strings plotted perpendicular to the axis the default is to centre the string.
`gap.axis` | an optional (typically non-negative) numeric factor to be multiplied with the size of an ‘m’ to determine the minimal gap between labels that are drawn, see ‘Details’. The default, `NA`, corresponds to `1` for tick labels drawn parallel to the axis and `0.25` otherwise, i.e., the default is equivalent to
      perpendicular <- function(side, las) {
        is.x <- (side %% 2 == 1) # is horizontal x-axis
        ( is.x && (las %in% 2:3)) ||
        (!is.x && (las %in% 1:2))
      }
      gap.axis <- if(perpendicular(side, las)) 0.25 else 1
`gap.axis` may typically be relevant when `at = ..` tick-mark positions are specified explicitly.
`...` | other graphical parameters may also be passed as arguments to this function, particularly, `cex.axis`, `col.axis` and `font.axis` for axis annotation, i.e. tick labels, `mgp` and `xaxp` or `yaxp` for positioning, `tck` or `tcl` for tick mark length and direction, `las` for vertical/horizontal label orientation, or `fg` instead of `col`, and `xpd` for clipping. See `par` on these. Parameters `xaxt` (sides 1 and 3) and `yaxt` (sides 2 and 4) control if the axis is plotted at all. Note that `lab` will partial match to argument `labels` unless the latter is also supplied. (Since the default axes have already been set up by `plot.window`, `lab` will not be acted on by `axis`.)

### Details

The axis line is drawn from the lowest to the highest value of `at`, but will be clipped at the plot region. By default, only ticks which are drawn from points within the plot region (up to a tolerance for rounding error) are plotted, but the ticks and their labels may well extend outside the plot region. Use `xpd = TRUE` or `xpd = NA` to allow axes to extend further.

When `at = NULL`, pretty tick mark locations are computed internally (the same way `axTicks(side)` would) from `par("xaxp")` or `"yaxp"` and `par("xlog")` (or `"ylog"`). Note that these locations may change if an on-screen plot is resized (for example, if the `plot` argument `asp` (see `plot.window`) is set.)

If `labels` is not specified, the numeric values supplied or calculated for `at` are converted to character strings as if they were a numeric vector printed by `print.default(digits = 7)`.

The code tries hard not to draw overlapping tick labels, and so will omit labels where they would abut or overlap previously drawn labels. This can result in, for example, every other tick being labelled. The ticks are drawn left to right or bottom to top, and space at least the size of an ‘m’, multiplied by `gap.axis`, is left between labels. In previous R versions, this applied only for labels written parallel to the axis direction, hence not for e.g., `las = 2`. Using `gap.axis = -1` restores that (buggy) previous behaviour (in the perpendicular case).

If either `line` or `pos` is set, they (rather than `par("mgp")[3]`) determine the position of the axis line and tick marks, and the tick labels are placed `par("mgp")[2]` further lines into (or towards for `pos`) the margin.

Several of the graphics parameters affect the way axes are drawn. The vertical (for sides 1 and 3) positions of the axis and the tick labels are controlled by `mgp[2:3]` and `mex`, the size and direction of the ticks is controlled by `tck` and `tcl` and the appearance of the tick labels by `cex.axis`, `col.axis` and `font.axis` with orientation controlled by `las` (but not `srt`, unlike S which uses `srt` if `at` is supplied and `las` if it is not). Note that `adj` is not supported and labels are always centered. See `par` for details.

### Value

The numeric locations on the axis scale at which tick marks were drawn when the plot was first drawn (see ‘Details’).

This function is usually invoked for its side effect, which is to add an axis to an already existing plot.

### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

### See Also

`Axis` for a generic interface.

`axTicks` returns the axis tick locations corresponding to `at = NULL`; `pretty` is more flexible for computing pretty tick coordinates and does not depend on (nor adapt to) the coordinate system in use.

Several graphics parameters affecting the appearance are documented in `par`.

### Examples

    require(stats) # for rnorm
    plot(1:4, rnorm(4), axes = FALSE)
    axis(1, 1:4, LETTERS[1:4])
    axis(2)
    box() #- to make it look "as usual"

    plot(1:7, rnorm(7), main = "axis() examples",
         type = "s", xaxt = "n", frame = FALSE, col = "red")
    axis(1, 1:7, LETTERS[1:7], col.axis = "blue")
    # unusual options:
    axis(4, col = "violet", col.axis = "dark violet", lwd = 2)
    axis(3, col = "gold", lty = 2, lwd = 0.5)

    # one way to have a custom x axis
    plot(1:10, xaxt = "n")
    axis(1, xaxp = c(2, 9, 7))

    ## Changing default gap between labels:
    plot(0:100, type="n", axes=FALSE, ann=FALSE)
    title(quote("axis(1, .., gap.axis = f)," ~~ f >= 0))
    axis(2, at = 5*(0:20), las = 1, gap.axis = 1/4)
    gaps <- c(4, 2, 1, 1/2, 1/4, 0.1, 0)
    chG <- paste0(ifelse(gaps == 1, "default:  ", ""),
                  "gap.axis=", formatC(gaps))
    jj <- seq_along(gaps)
    linG <- -2.5*(jj-1)
    for(j in jj) {
        isD <- gaps[j] == 1 # is default
        axis (1, at=5*(0:20), gap.axis = gaps[j], padj=-1, line = linG[j],
              col.axis = if(isD) "forest green" else 1, font.axis= 1+isD)
    }
    mtext(chG, side=1, padj=-1, line = linG -1/2, cex=3/4,
          col = ifelse(gaps == 1, "forest green", "blue3"))
    ## now shrink the window (in x- and y-direction) and observe the axis labels drawn

[Package graphics version 4.0.5 Index]