Question about plotting envelope of discrete signals

Hello there,

I used stem to generate a discrete und non-uniform sampled signals. The x-axis is time: 1,10,30,80,120 (days), and the y-axis is the parameter of a product. The y-values come from a datasheet and therefore is not a function of x.

I tried to plot an envelope of discrete y sequence using interp1, interp, envelope but couldn't make it. It seems that all of these MatLab functions require a function relationship between x and y? So how can I solve this problem, any ideas?

Thank you!!