\documentclass[tikz]{standalone}
\usepackage{tikz}

\begin{document}

\usetikzlibrary{datavisualization}
\begin{tikzpicture}

  \datavisualization data group {genes} = {
    data[set=gene1] {
      x, y
      0, 1,
      1, 2,
      2, 1.5
    }
    data[set=gene2] {
      x, y
      0, 1.5,
      1, 2.25,
      2, 1.75
    }
    data[set=gene3] {
      x, y
      0, 0.25,
      1, 0.26,
      2, 0.7
    }
    data[set=gene4] {
      x, y
      0, 0.5,
      1, 0.25,
      2, 1
    }
  };
  \datavisualization [
  school book axes, all axes={unit length=7.5mm},
  visualize as smooth line/.list={gene1, gene2, gene3, gene4},
  style sheet=strong colors,
  x axis={label=$t$},
  y axis={label={expression}}]
data group {genes};

\end{tikzpicture}

\end{document}