Please see the FAQ: What's a reproducible example (`reprex`) and how do I do one? Using a reprex, complete with representative data will attract quicker and more answers. This question is more conceptual than coding, however, so it isn't strictly needed.
Let's consider the object returned by plot. It's a two-dimensional representation of data along two axes, x and y. By convention, in lm models the dependent variable is plotted along the x axis, so that it's possible to see the return value of the function lm and its arguments.
This works well with a dependent variable for y and a single dependent variable for x. For every one unit increment of x what is the change in y is the question it answers.
But what happens with six independent variables, that have different scales plus one is categorical and the others continuous?
Two dimensions won't cut it. The vector through 6-space is the trend line and that's difficult to show in 2-space.
In my admittedly limited understanding of intermediate to advanced multiple linear regression is that the alternative is to display the partial residuals piecewise.
A good resource (so far) for me Harrel's Regression Modeling Strategies.