Make color Argument Optional Across Backends

[isofer]

Some plotting functions (e.g., line_xy) do not require a color argument when using the Matplotlib backend, but the same functions do require it when using the Plotly backend. This inconsistency can lead to confusion and extra boilerplate for users switching between backends. It would be helpful to make the color argument optional in the Plotly backend as well, ensuring consistent behavior and a smoother user experience across all supported backends.