Text alignment below horizontal axis with Tikz

Question

I'm trying to align the labels a and b under the x axis to the same baseline. The alignment is wrong on the left-hand side of the picture. It is correct on the right-hand side, but isn't there a simpler way to achieve this (that is, without finding a value for below= by trial and error)?

Left-hand side:

      \begin{tikzpicture}[scale=0.65]%
      \draw [<->] (7,0) node[below left]{$x$} -- (0,0) -- (0,7);
      \draw [thick] (0,0) -- (6.5,6.5);%
      \node [below] at (2,0) {$a$};%
      \node [below] at (5,0) {$b$};%
      \end{tikzpicture}%

Right-hand side:

      \begin{tikzpicture}[scale=0.65]%
      \draw [<->] (7,0) node[below left]{$x$} -- (0,0) -- (0,7);
      \draw [thick] (0,0) -- (6.5,6.5);%
      \node [below=2.735] at (2,0) {$a$};%
      \node [below] at (5,0) {$b$};%
      \end{tikzpicture}%

Answer 1

One way of doing this is to add a \vphantom{b} in the a node. This inserts a zero-width rule the height of b, so basically makes the height of the a node the same as the b node.

Another option is to place the node relative to the bottom of the b node. First give the b node a name/label (\node [below] (b) at (5,0) {$b$};), then place the second node with \node [above] at (b.south -| 2,0) {$a$};.

A third choice is to set the text height to be the height of a $b$. You can get that height width the \heightof macro, so set text height=\heightof{$b$}.

\documentclass[tikz,border=3mm]{standalone}
\begin{document}
\begin{tikzpicture}[scale=0.65]%
\draw [<->] (7,0) node[below left]{$x$} -- (0,0) -- (0,7);
\draw [thick] (0,0) -- (6.5,6.5);
\node [below] at (2,0) {$a\vphantom{b}$};
\node [above] at (b.south -| 3,0) {$a$};
\node [below] at (5,0) {$b$};
\node [below,text height=\heightof{$b$}] at (4,0) {$a$};
\end{tikzpicture}
\end{document}