daten1 = {{2, 0.3}, {4, 2.0}, {7, 8.1}, {9, 46.2},
          {11, 107.1}, {14, 105.1}, {16, 96.0},
          {18, 82.8}, {21, 81.7}, {23,79.1}, {25,88.2}}; 
daten1Rund = Interpolation[daten1];
daten1Ableitung[t_] = D[daten1Rund[t],t];
bild = Plot[daten1Ableitung[t],{t,2,25},
            ImageSize->{400,300}];