@maple2015 Here is your first line (the one using curve) back. As you see I replaced your PLOT with plots:-display and added a label. plots:-display puts the plot structure onto the screen, which in practice means you can wrap it around almost any plotting command and add options the plotting command you use does not know how to process. In addition, plots:-display accepts a list of plot structures and thus put several curves onto one graph. It is a very versatile command that is well worth studying in depth (perusing its Help file).
SN := curve([[3, (4.566256-4.544647)/(4.544647)], [4, (4.544933-4.544647)/(4.544647)], [5, (4.544653-4.544647)/(4.544647)], [6, (4.544649-4.544647)/(4.544647)], [7, 0]]): plots:-display(SN,axis=[mode=log],labels=['x','y']);
Since you are using log plots there is one word of warning here: I found out just the other day that an old bug with log plots has surfaced again in Maple 2015: if you plots:-display a log plot it sometimes shows up linear only, independent where (and how often) you specify mode=log for the axis you want logarithmically. Jeez, Maplesoft, wtf??? Anyway, a possible solution is to use plots:-setoptions(axis=[mode=log]) before the plotting commands. Experiment, it does not always work for me either.