The ornamental design for a chromatograph, as shown and described.
FIG. 1 is a front left perspective view of a chromatograph shown with the third and fourth modules removed in order to show structure that is hidden from view when all of the modules are attached;
FIG. 2 is a right view thereof with four modules;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a left view thereof; and,
FIG. 6 is a front view thereof.
The dash-dash broken lines in the figures and the lower opacity areas within the dash-dash lines show portions of the chromatograph that form no part of the claimed design.