Mix pt and cm units in Tikz?

Question

I have a plot with two horizontal lines. Both lines should have ticks, but only the lower line should have labels below the x-axis. Unfortunately, I don't know how to add ticks to the upper line, because the units are in cm and the ticks are added in pt. Do I need to convert the 1pt and -3pt to cm and put the absolute values there? I tried to mix using something like this \draw (2.5,15+1pt) -- (2.5,15-3pt) but this failed.

Here is the plot:

Code:

\begin{figure}
  \centering
    \begin{tikzpicture}[y=.2cm, x=.7cm]
        %axis
        \draw (1,0) -- coordinate (x axis mid) (18.9,0);
        \draw (1,15) -- coordinate (bla) (18.9,15);
        %ticks
        \draw (2.5,1pt) -- (2.5,-3pt)
            node[anchor=north] {1};
    %   \draw (2.5,1pt) -- (2.5,-3pt) <---- add here ticks on the upper horizontal line at the same x position as the lower line (1)
    \end{tikzpicture}
    \captionof{figure}{bla}
    \Description{bla}
    \label{fig:bla}
\end{figure}

Answer 1

The coordinates without numbers are in units of the x, y arguments of the tikzpicture (default 1 cm).

In this case, one possible approach is to use shifts applied to coordinates, like:

\draw ([yshift=1pt]2.5,15) -- ([yshift=-3pt]2.5,15)

When you mix pure numbers and units, the pure number is assumed to be in pt:

\documentclass[border=10pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[]
        %axis
        \draw [ultra thin, dotted] (0,0) grid (3,3);
        \draw (1,2) -- ++(2,0);
        \draw [red] ([yshift=5pt] 1,2) -- ++(2,0);
        \draw [blue] (1,2+5pt) -- ++(2,0);
        \node [circle, draw, green] at (1cm, 7pt){};
    \end{tikzpicture}
\end{document}

BTW, notice (for the next time) that this is a correct minimal working example (MWE)