Her original data showing the Cepheid period-luminosity relation. (Note this has been mislabeled with "LMC" and "SMC" -- the two data sets actually refer to the maximum and minimum brightness from the Cepheid's light curve.)
Sketch of the (modern) Cepheid Period Luminosity Relation.
Henrietta Leavitt writes :
The two resulting curves, one for maxima and one for minima, are surprisingly smooth, and of remarkable form. In Figure 2 ... a straight line can readily be drawn among each of the two series of points corresponding to maxima and minima, thus showing that there is a simple relation between the brightness of the variables and their periods ... Since the variables are probably at nearly the same distance from the Earth, their periods are apparently associated with their actual emission of light, as determined by their mass, density, and surface brightness.
Her results, including these words, were published by Edward C. Pickering. See Harvard College Observatory Circular, vol 173, p. 1, 1912.
The plot was taken from this site
Many portraits can be found here