Lowrance. Never heard of a graph, sonar unit, fish finder, etc. referred to as a “chart plotter” but there’s a first for everything!
”Graphs”, “sonar units” and “fish finders” whatever you want to call them, do not have map functions.
Chart plotters do not have sonar capabilities.
A lot of people like myself use dedicated units for each function. Mainly for the size of the display.
What you have is a called a “combo” unit. Both functions combined into one unit.
To answer your question, disk storage space on any unit is limited.
On some units the default is to over write existing data once full. Others give you the ability to just temporarily store data for a period of time before deleting.
I suspect your unit is set to delete tracks after so many days. Scroll through the menus looking for “ track” related options
