twoord.plot

0th

Percentile

Plot with two ordinates

Two sets of values are displayed on the same plot with different ordinate scales on the left and right.

Keywords
misc
Usage
twoord.plot(lx,ly,rx,ry,data=NULL,main="",xlim=NULL,lylim=NULL,rylim=NULL,
 mar=c(5,4,4,4),lcol=1,rcol=2,xlab="",lytickpos=NA,ylab="",ylab.at=NA,
 rytickpos=NA,rylab="",rylab.at=NA,lpch=1,rpch=2,
 type="b",xtickpos=NULL,xticklab=NULL,halfwidth=0.4,axislab.cex=1,
 do.first=NULL,...)
Arguments
lx,ly,rx,ry

y and optional x values for the plot

data

an optional data frame from which to obtain the above values

main

Title for the plot

xlim

optional x limits as in plot

lylim,rylim

optional y limits for the left and right axes respectively

mar

optional margin adjustment, defaults to c(5,4,4,4)

lcol,rcol

colors to distinguish the two sets of values

xlab

X axis label as in plot

lytickpos

Optional positions for the left axis tick labels.

ylab

Left Y axis label as in plot

ylab.at

Optional position for the left Y axis label

rytickpos

Optional positions for the right axis tick labels.

rylab

Right Y axis label

rylab.at

Optional position for the right Y axis label

lpch,rpch

plot symbols to distinguish the two sets of values

type

as in plot

xtickpos

Optional positions for x-axis tick labels.

xticklab

Optional labels for x-axis. Useful for things like dates.

halfwidth

Half the width of the bars in user units. The bars are centered on successive integers if no x values are supplied.

axislab.cex

Character expansion for the axis labels and tick labels.

do.first

Optional command(s) that will be executed immediately after the blank plot is displayed.

...

additional arguments passed to plot and points.

Details

twoord.plot automates the process of displaying two sets of values that have different ranges on the same plot. It is principally useful in illustrating some relationship between the values across the observations. It is assumed that the lx and rx values are at least adjacent, and probably overlapping.

It is best to pass all the arguments lx, ly, rx, ry, but the function will attempt to substitute sensible x values if one or two are missing.

If at least one of the type arguments is "bar", bars will be plotted instead of points or lines. It is best to plot the bars first (i.e. relative to the left axis) if the other type is points or lines, as the bars will usually obscure at least some of the points or lines. Using NA for the color of the bars will partially correct this. If both types are to be bars, remember to pass somewhat different x values or the bars will be overplotted.

Note that more values can be added to the plot using points or lines, but remember that these will be plotted relative to the left ordinate.

The do.first argument is useful for adding a background color or grid to the plot as shown in the first two examples.

Value

nil

Note

There are many objections to the use of plots with two different ordinate scales, and some of them are even sensible and supported by controlled observation. Many of the objections rest on assertions that the spatial arrangement of the values plotted will override all other evidence. Here are two:

The viewer will assume that the vertical position of the data points indicates a quantitative relationship.

To some extent. It is probably not a good idea to have the spatial relationship of the points opposed to their numerical relationship. That is to say, if one set of values is in the range of 0-10 and the other 20-100, it is best to arrange the plot so that the latter values are not plotted below the former. See the second example, which illustrates a method for separating the two series and offsetting the axes.

The viewer will assume that an intersection of lines indicates an intersection of values.

If the visual elements representing values can be arranged to avoid intersections, so much the better. Many people have no trouble distinguishing which visual elements are linked to which axis as long as they are both coded similarly, usually with colors and/or symbols. In the special case where there is an underlying relationship between the two such as the probability of that value occurring under some conditions, it may help to mark the point(s) where this occurs.

It may be useful to consider gap.plot or separate plots as an alternative.

See Also

plot

Aliases
  • twoord.plot
Examples
# NOT RUN {
 xval1 <- seq.Date(as.Date("2017-01-02"),
  as.Date("2017-01-10"), by="day")
 xval2 <- seq.Date(as.Date("2017-01-01"),
  as.Date("2017-01-15"), by="day")
 going_up<-seq(3,7,by=0.5)+rnorm(9)
 going_down<-rev(60:74)+rnorm(15)
 twoord.plot(2:10,going_up,1:15,going_down,xlab="Sequence",
  ylab="Ascending values",rylab="Descending values",lcol=4,
  main="Plot with two ordinates - points and lines",
  do.first="plot_bg();grid(col=\"white\",lty=1)")
 axis.Date(1,xval2)
 # now separate the lines
 twoord.plot(2:10,going_up,1:15,going_down,xlab="Sequence",
  lylim=range(going_up)+c(-1,10),rylim=range(going_down)+c(-10,2),
  ylab="Ascending values",ylab.at=5,rylab="Descending values",
  rylab.at=65,lcol=4,main="Plot with two ordinates - separated lines",
  lytickpos=3:7,rytickpos=seq(55,75,by=5),
  do.first="plot_bg();grid(col=\"white\",lty=1)")
 twoord.plot(2:10,going_up,1:15,going_down,xlab="Sequence",
  lylim=range(going_up)+c(-1,10),rylim=range(going_down)+c(-10,2),
  type=c("bar","l"),ylab="Ascending values",ylab.at=5,
  rylab="Descending values",rylab.at=65,
  main="Bars on left axis, lines on right axis",
  lytickpos=3:7,rytickpos=seq(55,75,by=5),
  lcol=3,rcol=4,do.first="plot_bg()")
 twoord.plot(2:10,going_up,1:15,going_down,xlab="Sequence",
  lylim=c(-3,8),rylim=c(50,100),type=c("l","bar"),
  ylab="Ascending values",rylab="Descending values",
  lytickpos=3:7,rytickpos=seq(55,75,by=5),ylab.at=5,rylab.at=65,
  main="Lines on left axis, bars on right axis",
  lcol=3,rcol=4,do.first="plot_bg(\"yellow\")")
 # histogram with density curve superimposed
 xhist<-hist(rnorm(100),plot=FALSE)
 xdens<-dnorm(seq(-3,3,by=0.05))
 twoord.plot(xhist$mids,xhist$counts,seq(-3,3,by=0.05),
 xdens,type=c("bar","l"),lcol=4,rcol=2,ylab="Counts",
 rylab="Density",main="Histogram and density curve",
 halfwidth=0.2,lylim=c(0,max(xhist$counts)+1),rylim=c(0,0.45),lwd=2)
# }
Documentation reproduced from package plotrix, version 3.7-4, License: GPL (>= 2)

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