Default plotting texts in italics

I simply created 2 series of data and used plot() function, and everything in the figure is in italics, even though I have set the parameters like "font.axis" to 1 (default, no italics, no bald, no Greek). Neither did I change the settings through par() function (in fact, it still persists even though I used par() to set the parameters to 1).

x = c(1,2,3,4)
y = c(1,2,3,4)

After testing, other default plotting fonts (Times New Roman and Courier) would be fine and generate images without italics, so I suppose maybe R Arial is broken for my computer. I deleted my current version of RStudio and installed the newest one, but the problem persists.

sessionInfo() output: Ubuntu Pastebin