To get help:
1.) You might explain what ELISA is. Here, done it for you
2.) Show us what you tried. Provide a data sample.
3.) An obvious method is to plot multiple y values is to plot the y value points that share a common x value in a different color. What you do depends on what your data set shows.
4.) This could be done using a Canvas or using an extension. What you do depends on what you will do with the data. If you use a Canvas, your curve drawing is limited to connect the dots. Some javascript techniques might allow curve fits. See Juan-Antonio’s javascript tricks Proposals of math codes. Area polygon. Vertices rectangle. Linear regression. Decimal IP
Regards,
Steve