Stress-Strain Curves (Compression):

For the ductile materials stress strain curves in compression are similar to those in tension at least up to the yield point for all the practical purposes. As tests in tension are easy   to make, the results derived from tensile curves are relied upon for the ductile materials in compression. 

Brittle materials are having compression stress strain curves usually of the same form as the tension test but the stresses at various points (Limit of proportionality, ultimate etc) are generally considerably different.