How to change subcaption numbering?

Question

I'm currently using this code

\documentclass{article}

\usepackage{caption}
\usepackage{subcaption}
\usepackage{graphicx}

\renewcommand{\thefigure}{\Roman{figure}}
\captionsetup[figure]{labelsep=period}

\begin{document}

\begin{figure}[!htb]
\caption{Optimal Claims}\label{optimalclaims}
\centering
\begin{subfigure}[H]{0.49\textwidth}
    \includegraphics[width=\textwidth]{Plot1.pdf}
    \medbreak
    \subcaption{X}\label{our}
\end{subfigure}
\hfill
\begin{subfigure}[H]{0.49\textwidth}
    \includegraphics[width=\textwidth]{Plot2.pdf}
    \medbreak
    \subcaption{Y}\label{mascu}
\end{subfigure}
\end{figure}
\end{document}

Which generates this output (when running this MWE on your computer, choose any two images, name them Plot1.pdf and Plot2.pdf and save them in the same folder as the main .tex file; it should compite without problems):

I have been trying to modify the numbering of the subcaptions by looking at this link, but with no success. Instead of (a) X and (b) Y, I would like to get I.a. X and I.b. Y. Can anybody please help me achieve so?

Thank you all very much in advanced for your time and effort.

Answer 1

(Remark: I posted the following answer based on the information given in the OP's MWE and, in particular, the fact that the article document class was supposed to be in use. The OP has since informed me -- see one of the comments below -- that he/she actually uses the scrartcl instead of the article document class. In consequence, my answer doesn't solve his/her actual typesetting issue.)

---

Something like this?

\documentclass{article}
\usepackage[demo]{graphicx} % remove 'demo' option in real document

\usepackage{subcaption} % 'caption' package is loaded automatically    
\captionsetup[figure]{labelsep=period}
\renewcommand{\thefigure}{\Roman{figure}}
\captionsetup[subfigure]{labelformat=simple} % default is 'parens'
\renewcommand\thesubfigure{\thefigure.\alph{subfigure}.}

\begin{document}

\begin{figure}[!htb]
\caption{Optimal Claims}\label{optimalclaims}
\begin{subfigure}[b]{0.49\textwidth}
    \includegraphics[width=\textwidth]{Plot1.pdf}
    \medbreak
    \caption{X}\label{our}
\end{subfigure}%
\hfill % to maximize separation between the subfigures
\begin{subfigure}[b]{0.49\textwidth}
    \includegraphics[width=\textwidth]{Plot2.pdf}
    \medbreak
    \caption{Y}\label{mascu}
\end{subfigure}
\end{figure}

\end{document}

---

Addendum: If you plan to create cross-references to subfigures, it would be best if you replaced the two lines of code

\captionsetup[subfigure]{labelformat=simple} % default is 'parens'
\renewcommand\thesubfigure{\thefigure.\alph{subfigure}.}

with

\DeclareCaptionSubType*[alph]{figure}
\captionsetup[subfigure]{labelformat=simple,labelsep=period}

See section 5 of the user guide of the subcaption package for the commands \DeclareCaptionSubType and \DeclareCaptionSubType*. With this change, the period at the end of the composite subcaption number isn't a part to the cross-reference output.