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, col = NULL, hadj = NA, padj = 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 vector of character strings or expressions to be placed at the tickpoints. If this is specified as strings or expressions, `at` should be supplied and they should be the same length.
`tick` | a logical value specifying whether tickmarks should be drawn
`line` | the number of lines into the margin which the axis will be drawn. If not `NA` this overrides the value of the graphical parameter `mgp[3]`. The relative placing of tickmarks and tick labels is unchanged.
`pos` | the coordinate at which the axis line is to be drawn: if not `NA` this overrides the values of both `line` and `mgp[3]`.
`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, lwd` | line type, width for the axis line and the tick marks.
`col` | color for the axis line and the tick marks. Here `NULL` means to use `par("fg")`, possibly specified inline.
`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.
`...` | other graphical parameters may also be passed as arguments to this function, particularly, `cex.axis`, `col.axis` and `font.axis` for axis annotation, `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`, 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 `xpd` is not accepted as clipping is always to the device region, and that `lab` will partial match to argument `labels` unless the latter is also supplied. (Since the default axes has 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. 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.

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` 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’ is left between labels.)

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`, 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. 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

    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))

[Package graphics version 2.3.1 Index]