Okay, I understand. It looks like you're grappling with a pretty common headache in ggplot2: how to make two dashed lines of different colors clearly visible when they overlap, especially if you want them to have the exact same dash pattern but just... offset a bit, so the dashes don't land right on top of each other. That's a totally reasonable thing to want!

Here's the gist of what's going on and what you can (and can't easily) do, based on how R and ggplot2 handle lines:

The Not-So-Great News: Direct "Dash Phase" Control Isn't Really a Thing in R/ggplot2

Unfortunately, R's graphics engine (which ggplot2 uses under the hood) doesn't have a simple switch or parameter to say, "Hey, for this second blue line, start drawing the dashes just a little bit later along the line than you did for the red one."

• How Linetypes Work: When you define a linetype (like 'dashed' or even a custom pattern like "44" which means "4 units on, 4 units off"), you're defining the repeating pattern itself, but not its starting point or "phase" along the line.

• Device-Dependent "Units": Those "units" in custom linetypes are also a bit tricky. They are generally proportional to the line width (linewidth or lwd), but how a basic unit (like for lwd=1) translates to actual pixels or points can vary depending on where you're looking at the plot (your RStudio plot pane, a PNG file, a PDF, etc.). This makes trying to create a "shifted" pattern by just slightly changing the numbers in the hex string (e.g., line1 = "44", line2 = "1344") very unreliable and not a general fix – it might look okay on your screen but break when you save it.

So, What Are Your Best Bets?

Since a direct "phase" control is off the table within R's standard graphics, here are the most practical ways people tackle this:

1. The "True but Complicated" Fix: SVG Export and stroke-dashoffset

   • If you absolutely need that perfect phase control, the way to get it is to step outside R's direct rendering. The SVG (Scalable Vector Graphics) format does support an attribute called stroke-dashoffset which does exactly what you want.

   • The process would be: create your ggplot2 plot -> export it to SVG (e.g., using the gridSVG package) -> then programmatically edit that SVG file to add the stroke-dashoffset to one of your lines -> then use the modified SVG.

   • This is powerful but definitely more involved than a simple ggplot2 tweak.

2. Making Lines Visually Distinct with ggfx (A Good ggplot2-Friendly Option)

   • The ggfx package lets you add some cool visual effects to ggplot2 layers. While it won't shift the dash phase, it can make one line stand out from the other.

   • A good candidate is with_outer_glow(). You could apply a subtle glow to one of your lines. It doesn't change the dashes themselves but makes the line appear slightly "thicker" or haloed, helping to distinguish it.

   • Conceptual Example:

     R

     # install.packages("ggfx") # if you haven't already
     library(ggplot2)
     library(ggfx)
     
     ggplot(data.frame(x=1:5,y=1:5)) +
       geom_point(aes(x=x,y=y)) +
       geom_abline(slope=1, intercept=1, linetype='dashed', color='red', linewidth=2) +
       with_outer_glow( # Apply glow to the blue line
         geom_abline(slope=1, intercept=1, linetype='dashed', color='blue', linewidth=2),
         colour = "lightblue", # Or even "blue" for a softer edge
         sigma = 1.5,        # Adjust for how much glow
         expand = 1          # Might need to tweak this
       )
     
     

   • This keeps you within the ggplot2 world and can be quite effective.

3. The Standard Workarounds (Which You Mentioned Wanting to Avoid, But Just for Completeness):

   • Transparency (alpha): geom_abline(..., alpha = 0.5) on the top line. Simple, but leads to mixed colors where dashes overlap.

   • Slightly Different Linetypes: Your example of a solid red line under a dashed blue line is one way. Or, using two different dash patterns (e.g., linetype = "dashed" vs. linetype = "longdash" or linetype = "dotdash"). This is often the most straightforward if you can accept a slight difference in the dash appearance.

   • Slight Positional Offset (If Your Data Allows): If it doesn't misrepresent your data, you could add a tiny numerical offset to the intercept of one of the lines so they aren't perfectly on top of each other.

Regarding That "Red Protruding" Antialiasing Thing:

Yeah, that little artifact where one color seems to "bleed" or "protrude" around the edge of another when they're close or overlapping is usually due to antialiasing. Antialiasing smooths out jagged edges, but how different graphics devices (your screen, PNGs, PDFs) do this can vary, and sometimes it results in these subtle visual quirks.

• Tip: Try saving your plot using a high-quality graphics device, like those provided by the ragg package (e.g., ragg::agg_png()) or Cairo-based devices (cairo_pdf(), cairo_png()). These often have better antialiasing and might reduce that effect.

In a Nutshell:

It stinks that there isn't a simple dash_phase argument in ggplot2. For making overlapping dashed lines distinct when you want the same dash pattern:

• ggfx (like with_outer_glow) is probably your best bet for a ggplot2-based solution that adds visual distinction without altering the line path or trying to simulate phase in an unreliable way.

• If you need true phase control, SVG export is the way, but it's a bigger lift.

Hope this helps clear things up, even if it's not the magic bullet solution we all wish R had for this one!