How to plot a horizontal quadratic function?

Question

I am trying to plot a "sideways" or horizontal quadratic function in R but am running into the issue of having sqrt(-x) which is a problem of course.

eq1 = function(x){ -60*(sqrt(1-x)-1) }
eq2 = function(x){ 60*(sqrt(1-x)+1) }

plot(eq1, 0, 100, add=TRUE, xlim=c(0,1), ylim=c(0,100))
plot(eq2, 0, 100, add=TRUE)

Here is an example of the output plot

And the output from the R Console:

> eq1 = function(x){ -60*(sqrt(1-x)-1) }
> eq2 = function(x){ 60*(sqrt(1-x)+1) }
> 
> 
> plot(eq1, 0, 100, add=TRUE, xlim=c(0,1), ylim=c(0,100))
Warning messages:
1: In curve(expr = x, from = from, to = to, xlim = xlim, ylab = ylab,  :
  'add' will be ignored as there is no existing plot
2: In sqrt(1 - x) : NaNs produced
> plot(eq2, 0, 100, add=TRUE)
Warning message:
In sqrt(1 - x) : NaNs produced

If I understand correctly, there are domain restrictions on a quadratic function like this. If so, is there a way to incorporate them into the function as defined in R? Or is there are better way to go about drawing this function?

Answer 1

You need to correctly define the range you want to plot the function on with the x argument to plot.function:

plot(eq1, seq(0, 1, 0.01), xlim=c(0,1), ylim=c(0,100))
plot(eq2, seq(0, 1, 0.01), add=TRUE)

Actually, plot.function is smart enough that even without a range it will fix that for you:

plot(eq1, xlim=c(0,1), ylim=c(0,100))
plot(eq2, add=TRUE)

(With no warnings.)