Fix replacing base::.External.graphics

[renkun-ken]

What problem did you solve?

Closes #624

This PR fixes a regression introduced in #602 where base::.External.graphics is not successfully replaced to trigger graphics replay so that functions like abline(), points() could cause the plot to refresh.

(If you do not have screenshot) How can I check this pull request?

When each the following lines is run, the png plot file is refreshed in vscode.

plot(rnorm(100))
abline(h = 0, col = "red")
abline(v = 0, col = "blue")