Years ago I found this solution to typeset systems of equation within Mathematica:

Grid[{{PasteButton[
    DisplayForm[
     StyleBox[
      RowBox[{StyleBox["{", SpanMaxSize -> \[Infinity]], 
        GridBox[{{"\[Placeholder]"}, {"\[Placeholder]"}}, 
         ColumnAlignments -> Left]}], ShowAutoStyles -> False]]], 
   PasteButton[
    DisplayForm[
     StyleBox[
      RowBox[{StyleBox["{", SpanMaxSize -> \[Infinity]], 
        GridBox[{{"\[Placeholder]"}, {"\[Placeholder]"}, {"\
\[Placeholder]"}}, ColumnAlignments -> Left]}], 
      ShowAutoStyles -> False]]], 
   PasteButton[
    DisplayForm[
     StyleBox[
      RowBox[{StyleBox["{", SpanMaxSize -> \[Infinity]], 
        GridBox[{{"\[Placeholder]"}, {"\[Placeholder]"}, {"\
\[Placeholder]"}, {"\[Placeholder]"}}, ColumnAlignments -> Left]}], 
      ShowAutoStyles -> False]]]}}, Spacings -> 0]

Hi Jay,

Can you please provide an example? What is "a two-column step-wise function"? For "a multi-line system of equations with a left brace" do you mean

f[x_] := Piecewise[{{Sin[x]/x, x < 0}, {1, x == 0}}, -x^2/100 + 1];
Plot[f[x], {x, -Pi, Pi}, PlotLabel -> f[x]]

You can provide any expression to PlotLabel, by default its TraditionalForm will be displayed.

If you are familiar with LaTeX then you can use the MaTeX package for typesetting.

Needs["MaTeX`"];

label = MaTeX["\\left\\{
  \\begin{array}{@{}ll@{}}
    xy^2 = 5 \\\\
    x^3 - y = 10 \\\\
    y \\leq 0
  \\end{array}\\right.", Magnification -> 1.25]

an use PlotLabel -> label.

I did a search on MSE and found this, which is much easier if you have a list of expressions.

Jay,

Did you take a look at the MSE link at the bottom of my answer? It has several different solutions to this problem.

Hi Jay,

Using the Format definition of bracket from that post

Format[bracket[obj_]] := Style[DisplayForm@RowBox[{"{", obj}], SpanMaxSize -> Infinity]

label = {Defer[xy^2 = 5], Defer[x^3 - y = 10], Defer[y <= 0]} // Column // bracket

or

label = HoldForm /@ Unevaluated@{xy^2 = 5, x^3 - y = 10, y <= 0} // Column // bracket

Then use it for PlotLabel