A cell array containing lots of scalar arrays indicates that your data is arranged sub-optimally. You would be much better off using a table, or perhaps two separate homogenous vectors (i.e. a datetime array and a numeric array).
Converting to two homogenous arrays is probably the easiest way to plot the existing (badly designed) data:
dt = [index{2:end,1}];
ix = [index{2:end,2}];
plot(dt,ix)
