When in one's portfolio screen, have an extra column that would show Gain/Loss percentage over different periods. Days, weeks, months.
Have an extra column in the portfolio screen which displays the percentage gain or loss (ROI) against each stock for different periods. Have the period selected by right clicking the column heading and choosing preset periods (e.g. 1,2,3,5,10 days, 1, 2,3, months, last quarter, last half year etc. or leave a space where one can simply enter the period required). At the moment one only has the ROI from date of purchase or from date of portfolio inception)