fix(plot-style): keep custom lineweights at full precision when saving

Saving a plot style table wrote every value of its custom lineweight table
with two decimals, so a pen such as 0.035 mm came back as 0.04 mm after a
save and reload, and every color using it plotted with the wrong weight.

Write the lineweights in full, through the same formatting the scale factor
already uses, which keeps the decimal point on a whole value.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/io/plot_style.rs

@@ -357,13 +357,8 @@ impl PlotStyleTable {
         let description = self.description.replace(['\r', '\n'], " ");
         s.push_str(&format!("description=\"{description}\n"));
         s.push_str("aci_table_available=TRUE\n");
-        // The factor scales every lineweight when applied, so write it in full,
-        // keeping the decimal point on a whole value.
-        let mut scale_factor = self.scale_factor.to_string();
-        if scale_factor.chars().all(|c| c.is_ascii_digit() || c == '-') {
-            scale_factor.push_str(".0");
-        }
-        s.push_str(&format!("scale_factor={scale_factor}\n"));
+        // The factor scales every lineweight when applied, so write it in full.
+        s.push_str(&format!("scale_factor={}\n", full_real(self.scale_factor)));
         s.push_str(&format!(
             "apply_factor={}\n",
             if self.apply_factor { "TRUE" } else { "FALSE" }
@@ -420,7 +415,7 @@ impl PlotStyleTable {
         }
         s.push_str("}\ncustom_lineweight_table{\n");
         for (index, weight) in self.lineweights.iter().enumerate() {
-            s.push_str(&format!(" {index}={weight:.2}\n"));
+            s.push_str(&format!(" {index}={}\n", full_real(*weight)));
         }
         s.push_str("}\n");
         s
@@ -512,6 +507,15 @@ fn adler32(bytes: &[u8]) -> u32 {
     (b << 16) | a
 }
 
+/// A real number written in full, keeping the decimal point on a whole value.
+fn full_real(value: f32) -> String {
+    let mut text = value.to_string();
+    if text.chars().all(|c| c.is_ascii_digit() || c == '-') {
+        text.push_str(".0");
+    }
+    text
+}
+
 fn packed_rgb([r, g, b]: [u8; 3]) -> i32 {
     u32::from_be_bytes([0xC2, r, g, b]) as i32
 }
@@ -780,4 +784,16 @@ mod round_trip_tests {
         assert!(table.to_text().contains("\nscale_factor=1.0\n"));
         assert_eq!(saved_and_reloaded(&table).scale_factor, 1.0);
     }
+
+    /// Saving keeps each custom lineweight as the table holds it. They used to
+    /// be cut to two decimals, so a 0.035 mm pen came back as a 0.04 mm one.
+    #[test]
+    fn custom_lineweights_survive_a_save() {
+        let mut table = PlotStyleTable::identity("pens.ctb");
+        table.lineweights[1] = 0.035;
+        table.aci_entries[1].lineweight = 1;
+        let reloaded = saved_and_reloaded(&table);
+        assert_eq!(reloaded.lineweights, table.lineweights);
+        assert_eq!(reloaded.resolve_lineweight(1), Some(0.035));
+    }
 }