devtools memory section issues

i wanted to use memory trace tool in devtools and stumbled upon these issues

1. ***(see video)*** chart sometimes does not correspond with memory values. on the video see on the `graphics` parameter, but if i recall correctly, i also saw `other` showing this behavior - this is applicable to both `debug` and `profile` modes
2. ***(see video)*** yellow selection bar position is starting to drawn incrorrectly after the chart fills the screen entirely, but numbers are shown "correctly". to speed up the reproduction process you can zoom in the page
3. bar position should be drawn immediately after click. currently it feels really undersponsive and makes you feel really uncomfortable during its usage
4. it's really hard to distungish different parameters on chart
5. legend colors do not match colors on chart:
* `graphics` is orange in legend, yellow on chart (when it's not overlayed, see the first video)
* `java` has entirely different green hue
* `stack` is grey on chart, white in legend

reproduction app with video i used to test https://github.com/nt4f04unds-archive/issues/tree/flutter_78128

flutter doctor -v

```
[✓] Flutter (Channel nt4f04und, 2.1.0-12.0.pre, on Microsoft Windows [Version 10.0.19041.867], locale ru-RU)
• Flutter version 2.1.0-12.0.pre at c:\dev\src\flutter
• Framework revision 698d438960 (5 days ago), 2021-03-09 07:53:21 +0100
• Engine revision 2441c476a6
• Dart version 2.13.0 (build 2.13.0-116.0.dev)

[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
• Android SDK at C:\Users\danya\AppData\Local\Android\sdk
• Platform android-30, build-tools 30.0.2
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
• All Android licenses accepted.

[✓] Chrome - develop for the web
• Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[!] Visual Studio - develop for Windows (Visual Studio Community 2019 16.7.7)
• Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community
• Visual Studio Community 2019 version 16.7.30621.155
• Windows 10 SDK version 10.0.18362.0
✗ Visual Studio is missing necessary components. Please re-run the Visual Studio installer for the "Desktop
development with C++" workload, and include these components:
MSVC v142 - VS 2019 C++ x64/x86 build tools
- If there are multiple build tool versions available, install the latest
C++ CMake tools for Windows
Windows 10 SDK

[✓] Android Studio (version 4.0)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin version 50.0.1
• Dart plugin version 193.7547
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[✓] VS Code (version 1.54.2)
• VS Code at C:\Users\danya\AppData\Local\Programs\Microsoft VS Code
• Flutter extension version 3.20.0

[✓] Connected device (4 available)
• Redmi Note 5 (mobile) • 40c9f14 • android-arm64 • Android 9 (API 28)
• Windows (desktop) • windows • windows-x64 • Microsoft Windows [Version 10.0.19041.867]
• Chrome (web) • chrome • web-javascript • Google Chrome 89.0.4389.82
• Edge (web) • edge • web-javascript • Microsoft Edge 89.0.774.50

! Doctor found issues in 1 category.
```

https://user-images.githubusercontent.com/39104740/111054800-98d97300-8480-11eb-9339-c4cc043132c2.mp4

https://user-images.githubusercontent.com/39104740/111054812-c0304000-8480-11eb-82b3-f4627d97b964.mp4

Start by running the DevTools memory trace tool with the linked reproduction app and compare both debug and profile modes with the attached videos. Investigate the chart values, selection-bar timing and position, parameter distinction, and legend colors; done means all five reported behaviors are corrected and verified during tracing.

索引モデルが issue の本文から書いたものです。