If we're going to design our own syntax to draw lines, I'll add a few things to the host language: First, a "point" data type, and a "line" data type. Most language can do this already.

Then I'd add some special syntax to denote points. Like one of those:

  (a, b) -- ordinary tuple.  Of the shelf in ML and Haskel.
  [a, b] // may work in C and C++
  (a; b) // Alternative syntax

We also need an operator to draw the line. Possibly a triple dash:

  (a, b) --- (c, d)

And of course this can be combined together (here in my custom version of C):

  Point p = (a; b);
  Line  l = p --- (c; d);

Now we need a way to actually draw the line. As a side effect. It could be a procedure, but when there's one line, there will be others. So, we often need to draw a list of lines. This syntax might work:

  draw {
    p      --- (c; d);
    (e; f) --- (g;h);
    (g; h) --- p;
  }

The semantics would be what you'd expect, with the possible benefits of some optimisation (some constructors can be avoided, the data can be packed before we draw all the lines at once…)

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Now If I get to really try that in a real-world project, I'll probably find many ways to improve this lousy design.