Help with plotting the Date as a continuous effect

[kenyonanna]

Hello,

I am a master's student and relatively new to R and MixSIAR. For my master's dissertation, I am investigating different pollution sources into the River Thames (my mixture), focusing on two sources: raw sewage and treated wastewater.

I have a large dataset from the River Thames spanning several years, and I want to analyse how the relative contributions of raw sewage and treated wastewater have changed over time. My goal, therefore, is to plot the date as a continuous effect. However, I'm facing an issue: my understanding is that the continuous effect must be numeric. This means I cannot plot dates on the x-axis unless they are in Julian date format.

Despite this, I have seen others use dates on the x-axis, as shown in the attached image. How did they manage to do this?

Please help me. Thank you in advance