I want to plot a logistic regression curve of my data, but whenever I try to my plot produces multiple curves. Here's a picture of my last attempt:

last attempt

Here's the relevant code I am using:

fit = glm(output ~ maxhr, data=heart, family=binomial)
predicted = predict(fit, newdata=heart, type="response")
plot(output~maxhr, data=heart, col="red4")
lines(heart$maxhr, predicted, col="green4", lwd=2)

My professor uses the following code, but when I try to run it I get an error on the last line saying that the x and y lengths do not match:

# fit logistic regression model
fit = glm(output ~ maxhr, data=heart, family=binomial)
# plot the result
hr = data.frame(maxhr=seq(80,200,10))
probs = predict(fit, newdata=dat, type="response")
plot(output ~ maxhr, data=heart, col="red4", xlab ="max HR", ylab="P(heart disease)")
lines(hr$maxhr, probs, col="green4", lwd=2)

Any help would be appreciated.

**Edit:**

As requested, reproduceable code using the mtcars dataset:

fit = glm(vs ~ hp, data=mtcars, family=binomial)
predicted= predict(fit, newdata=mtcars, type="response")
plot(vs~hp, data=mtcars, col="red4")
lines(mtcars$hp, predicted, col="green4", lwd=2)