Any moment-rate tensor can be expressed as , where MT is the total scalar moment and M^ is the source mechanism, a symmetric tensor of unit Euclidean norm describing the geometry of the source. We present a method for the graphical display of the second-order tensor M^ and its uncertainties, the latter specified by a fourth-order covariance tensor, var[M^]. The display allows the statistical significance of non-double-couple components to be tested by inspection. It is also useful in evaluating the frequency dependence of the source and assessing differences among moment-tensor estimates. We provide a detailed description of the plotting method and some examples of its application to source-mechanism spectra recovered from IDA data, including an earthquake in the Kermadec Islands that has a large non-double-couple component.