The older, less accurate data is shown in red. The more accurate data are shown in blue, plus the data point in black.
Note that the error bars of blue and black data points all straddle the line denoting the change in the Fine Structure Constant, alpha, being equal to zero.
The Fine Structure constant, alpha, includes terms representing the speed of light and the charge of an electron. Ergo, if the speed of light has changed during the history of the Universe, it should manifest itself as a change in alpha, as observed at various redshift distances corresponding to earlier eras in the history of the Universe.
This data, published in December 2005, is compatible with there having been no change at all in alpha, and hence the speed of light, over cosmological time scales.