feat: memory and swap graphs

the values are in the SysinfoSnapshot verbatim,
in case they might be displayed in another way (textually)

src/sysinfo_snapshot.rs

@@ -2,6 +2,11 @@
 pub struct SysinfoSnapshot {
     pub cpu_total: f32,
     pub cpus: Vec<f32>,
+
+    pub memory: u64,
+    pub memory_total: u64,
+    pub swap: u64,
+    pub swap_total: u64,
 }
 
 impl From<&sysinfo::System> for SysinfoSnapshot {
@@ -9,6 +14,11 @@ impl From<&sysinfo::System> for SysinfoSnapshot {
         Self {
             cpu_total: value.global_cpu_usage(),
             cpus: value.cpus().iter().map(|cpu| cpu.cpu_usage()).collect(),
+
+            memory: value.used_memory(),
+            memory_total: value.total_memory(),
+            swap: value.used_swap(),
+            swap_total: value.total_swap(),
         }
     }
 }