Ok, so I made a graph in LibreOffice Calc and i wanted to change the legend to write some chemical cmpounds but, as it turns out, i couldn’t seem to. here’s how it looked:
As you can see, Na2S2O3 and HCl looks horrible!
pliz help 
I cannot confirm the behavior. If I use small and capital letters in the table header, they are shown small and capital in the legend as well. Please have a look at the AutoCorrect options in Calc. I have disabled all but “Use Replacement table” and “URL recognition”; perhaps that interferes with the legend? It would be useful to get your document too, for to see, whether it shows wrong on other PCs too. I have attached my example ExampleSubscript.ods.
You should use the special characters from the subset “Superscripts and Subscripts” for the subscript in a chemical name. You have to define a cell style with a font, which has this subset, e.g. font “Dejavu serif”. The formatting of LibreOffice for subscript a character are not taken over to the legend and therefore the characters are all in the normal position in the legend. (That is the same problem as formatting of TOC entries in Writer.) It looks nicer, when you use these special characters.