Set linestyle via fmt in plot_two

[jeanconn]

Description

Set linestyle via fmt in plot_two

The previous fix to set the marker explicitly with 'marker' still had matplotlib throwing a warning because fmt is set and has default of 'o' in plot_date.

Testing

• [ n/a] Passes unit tests on MacOS, linux, Windows (at least one required)
• [x] Functional testing

This correctly makes the plot, still without the markers and with the linestyles... and doesn't throw any warnings in ska3-next or current ska3 flight.

[taldcroft]

I also searched the sot org for external calls to plot_two and found none (since the API is changing).