This is a historical question that needs some background to make sense. Let me start with the longer version of the question:

When did negative numbers, algebra and coordinate plane come together?

Here are some useful facts:

For a long time, even after recognition of negative numbers, there were mathematicians who actively tried to clean algebra from negative numbers (at least, up until late eighteen century).

For a long time, the use of letters in algebra was confined to positive quantities. Simply speaking $-a$ stood for a negative number by default (again, at least, up until late eighteen century).

And, this is the most surprising fact, and the reason that I ask this question. George Peacock (1791-1858), one of the pioneers of modern symbolic algebra, the person who gave an abstract treatment of negatives, and the person who gave a treatment of algebra in which letters could admit negatives as

*input*, when came to a geometric interpretation of imaginary numbers, treats coordinates in a way that only the first quarter is used (1830).

Let me finish this long post with a very concrete question somehow summarizing all my historical points: When you simply write $x+y=1$ as the equation of the line passing through $(0,1)$ and $(1,0)$, you work with the "standard" coordinate plane, and you know that $x$ and $y$ admit certain negative numbers as *input*. Historically, when did such an understanding come into play?

I hope the question makes sense at least for those who are interested in history of mathematics.