Specify decimal places in r plot text?

Question

I tried to format the label of my regression curve created in the base graphics system. Basically, this label extracts the slope, the intercept and the r-square values from variables. An example is as below: ``` plot(rnorm(10), type = "n", xlim = c(0, 100), ylim = c(0, 100)) text(x = 0, y = 100, adj = c(0, NA), bquote(paste(y == .(a) * x + .(b), ", R"^2 == .(r2))), cex = 0.7) ``` However, this generated a label that did not look very smart: ``` y = 1.159019x+-1.537708, R²=0.7924927 ``` What I would like is to round the figures to the second decimal place, i.e., ``` y = 1.16x-1.54, R²=0.79 ``` I have looked it up in both help documents for `text()` and `bquote()` but did not find much useful information. I have also tried to wrap the `.(a)`, `.(b)` and `.(r2)` in `format()` with the parameter `nsmall = 2`, but it did not work. Could someone kindly help me with this? Many thanks in advance! ps. I think there is a hidden quest in my question. In the above example, `…

Accepted Answer

For specifying the number of digits, use `round(a, digits=2)`. However, you can also use `sprintf` to deal with both the number of digits and the + or - in your equation: e.q. %+3.2f, where `%` forces the + or - sign in the equation and `3.2f` controls the number of digits, so this solves both your issues. I found a solution for the superscript problem in `sprintf` here: "B2 is the hex code for UTF-8 character = ^2 and \U is a control sequence that will call that character."

```
# data and regression
set.seed(1)
y = 1:10+rnorm(10)
x = 1:10
fit = lm(y~x)
b = coef(fit)[1]
a = coef(fit)[2]
r2 = summary(fit)$r.squared

# plot data and regression
plot(x, y)
abline(fit, col=2)

# add text to plot with legend() for convenient placement
legend('topleft', title='option 1', legend=sprintf("y = %3.2fx %+3.2f, R\UB2 = %3.2f", a, b, r2), bty='n', cex=0.7)

# if you prefer a pretty space between plus/minus and b:
if( b<0) {my_sign = ' - '; b = -b} else { my_sign= ' + '}
legend('bottomright', title='option 2', legend=sprintf("y = %3.2f x %s %3.2f, R\UB2 = %3.2f", a, my_sign, b, r2), bty='n', cex=0.7)
```

Specify decimal places in r plot text?

Solution

Problem