@SHIVAS The variable j is a for-loop index, and you're trying to use it outside of the for-loop. In this case, the value of j is 5 after the loop ends.

@Gabriel Barcellos Are you claiming that you see some difference between the two plots that you posted other than the font used for the tick labels and axis labels?

@C_R Thank you.

I made a small change to the MathML code in my Answer to improve the visibility of the numbers in the box. Would you please post a screenshot showing this (the plot, cursor, and box)? I don't know how to make a screenshot on my computer.

@SHIVAS Can't you see that there's a big empty space in the middle of your worksheet where there should be a pdsolve command? The error messages shown indicate that there was one there, and that it was at least syntactically correct. 

If you have a version that works, as you just said, post that.

@Suryakanth I think that you perhaps think that there's some small nuance of a plot command that can be "rectified" and then everything will work. But, actually, it's far more complicated than that. For example, there is no command named pdeplot. There is a command PDEplot, but it can only be used for a single first-order PDE. That's just an example; there are numerous other flaws. 

@nm When arrowsize is specified as a number, it is just a multiplier applied to the arclength of all arrows, and all arrows have the same arclength. This length is independent of the scaling of the axes or the implied 3rd dimension. This is different than arrowsize= 'magnitude', where the length of the arrow shows the field strength (which is the 3rd dimension). In the present case, it's the color that shows the field strength. So, I think that your concern about needing to change the arrowsize for different plots is not likley to actually be a problem.

@SHIVAS Thank you. I just put a link to this Question in that other thread.

@Suryakanth Here is a link to the nearly identical Question that I was sure that I'd seen: https://www.mapleprimes.com/questions/236809-Error-In-Ode-Plot-

@ Norma, have you tried the annotation option as I showed in my Answer? Although my code there is necessarily quite esoteric due to needing to use an essentially different computer language, MathML, for parts of it, I believe that it does exactly what you asked for: It gives the y-values for all curves in different colors in one "frame" for any given t value that your cursor is on (the cursor needs to be on any of the curves).

@C_R Left click on the plot to open its context menu, go to Probe Info, then check None.

@dharr In that case, I vote up. Thanks for clarifying the worksheet.

@dharr It's a good Answer, but one detail needs to be addressed: series has option system as well as option remember, which means that you can't rely on a remembered result staying remembered---they get cleared by garbage collection. 

@jrive You asked:

•  What do you mean by nontrivial solution?

Keep in mind that I'd misread that your intention was to solve for DC, T, and tau. In that case, I would consider 
DC = ton/(ton + toff) to be a trivial and undesirable solution (although technically correct), and I strove to avoid it.

@Gabriel Barcellos In that case, I don't understand why you called it an "Incorrect answer".

@jrive The confusion was my fault. I simply misread "in terms of" as "for". 

It would be clearer, at least for me, if this were stated at the bottom of the Question.