Enhance the doc of timing report with graphs

src/doc/src/reference/timings.md

@@ -12,13 +12,21 @@ filename, if you want to look at older runs.
 
 #### Reading the graphs
 
-There are two graphs in the output. The "unit" graph shows the duration of
-each unit over time. A "unit" is a single compiler invocation. There are lines
-that show which additional units are "unlocked" when a unit finishes. That is,
-it shows the new units that are now allowed to run because their dependencies
-are all finished. Hover the mouse over a unit to highlight the lines. This can
-help visualize the critical path of dependencies. This may change between runs
-because the units may finish in different orders.
+There are two tables and two graphs in the output. 
+
+The first table displays the build information of the project, including the 
+number of units built, the maximum number of concurrency, build time, and the 
+version information of the currently used compiler.
+
+![build-timing](../images/build-timing.PNG)
+
+The "unit" graph shows the duration of each unit over time. A "unit" is a single
+compiler invocation. There are lines that show which additional units are 
+"unlocked" when a unit finishes. That is, it shows the new units that are now 
+allowed to run because their dependencies are all finished. Hover the mouse over
+a unit to highlight the lines. This can help visualize the critical path of 
+dependencies. This may change between runs because the units may finish in 
+different orders.
 
 The "codegen" times are highlighted in a lavender color. In some cases, build
 pipelining allows units to start when their dependencies are performing code
@@ -28,6 +36,8 @@ units do not show when code generation starts).
 The "custom build" units are `build.rs` scripts, which when run are
 highlighted in orange.
 
+![build-unit-timing](../images/build-unit-timing.PNG)
+
 The second graph shows Cargo's concurrency over time. The background
 indicates CPU usage. The three lines are:
 - "Waiting" (red) --- This is the number of units waiting for a CPU slot to
@@ -36,6 +46,8 @@ indicates CPU usage. The three lines are:
   dependencies to finish.
 - "Active" (green) --- This is the number of units currently running.
 
+![cargo-concurrency-over-time](../images/cargo-concurrency-over-time.PNG)
+
 Note: This does not show the concurrency in the compiler itself. `rustc`
 coordinates with Cargo via the "job server" to stay within the concurrency
 limit. This currently mostly applies to the code generation phase.
@@ -49,3 +61,6 @@ Tips for addressing compile times:
 - Split large crates into smaller pieces.
 - If there are a large number of crates bottlenecked on a single crate, focus
   your attention on improving that one crate to improve parallelism.
+
+The last table lists the total time spent on "unit", "Codegen" time, and the 
+features that unit has.