[R] How to plot a legend centered only on the x axis

[Antonio Silva]

Thanks for you attention Mark. Your approach is really very interesting. I
will try to apply it in my code.

All the best

Antonio

2017-05-11 16:19 GMT-03:00 Marc Schwartz <marc_schwartz em me.com>:

>
> > On May 11, 2017, at 1:36 PM, Antonio Silva <aolinto.lst em gmail.com>
> wrote:
> >
> > Hello r-users
> >
> > I want to plot some barplots inside a looping with the legend placed
> > outside the plotting area.
> >
> > No matter the number of bars I want the legend to be placed centered on
> the
> > x-axis.
> >
> > See the example below
> >
> > for(i in 1:10) {
> > var_1 <- sample(1000:100000,sample(3:8,1))
> > ymax <- max(var_1)
> > b<-barplot(var_1,col="blue")
> > var_2 <- sample(1000:ymax,length(var_1))
> > lines(rowSums(b),var_2,type="o",col="red",pch=16)
> > par(xpd=TRUE)
> > legend(*1.1*
> > ,ymax*-0.072,c("var_1","var_2"),horiz=T,cex=1.3,bty="n",pch=
> c(15,16),col=c("blue","red"),lty=c(0,1),lwd=c(0,2),pt.cex=2)
> > readline(prompt="Press [enter] to continue")
> > }
> >
> > What I should use as x position in legend(x,y, ...), instead of *1.1*, to
> > have the legend centered on the x-axis?
> >
> > I would love to use something like legend(x="center",y=ymax*-0.072, ...
> but
> > it did not worked.co
> >
> > I appreciate any suggestions. Best regards.
> >
> > Antonio
>
>
> Hi,
>
> There is a level of magic that goes into the legend() code to determine
> the placement of the legend. You can take the time to review the code for
> the function, noting how, internally, the location of the box that contains
> the legend is determined.
>
> Somebody may have an easier way to do this, but one approach is to first
> call legend() but set 'plot = FALSE' and obtain the return values of the
> function for use in a second call that will draw the legend in the position
> you desire.
>
>
> Here is some basic information:
>
> # Draw the initial barplot
> barplot(1:5)
>
> # Call legend() the first time, but don't draw it
> # See ?legend for the Value section
> # Use "bottom" to get the legend drawn in the center of the x axis, but it
> # will be within the plot region by default.
> # Just save the 'rect' component of the returned value
> BOX <- legend("bottom", legend = c("Label 1", "Label 2"), plot =
> FALSE)$rect
>
> # This gives us the width and height of the box containing the legend
> # as well as the x,y coordinates of the upper left hand corner
> > BOX
> $w
> [1] 1.002451
>
> $h
> [1] 0.5872093
>
> $left
> [1] 2.598775
>
> $top
> [1] 0.5372093
>
>
> Now, using BOX$left, which defines the x axis value for the left side of
> the legend box that is centered, we can call legend() a second time and
> adjust the y axis value of the legend to place it outside the plot region,
> below the plot. Just be sure that your second call to legend() is exactly
> the same as the first call, other than setting legend to FALSE in the first
> case. Any change in legend content between the two calls will alter the
> size and position of the enclosing box.
>
> par(xpd = TRUE)
> legend(BOX$left, y = -0.25, legend = c("Label 1", "Label 2"))
>
>
> You can then adapt that basic approach to use in your code.
>
> As an FYI, see ?par and note "usr", which defines the coordinates of the
> plot region. If desired, you can get the midpoint of the x axis using:
>
>   mean(par("usr")[1:2])
>
> Regards,
>
> Marc Schwartz

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