Sequence-diagrams.md

+```mermaid
+sequenceDiagram
+    olqv.js->>+serverWsgi: server is up ?
+    serverWsgi->>olqv.js: yes
+    olqv.js->>+serverWsgi: get rendering capabilities
+    serverWsgi->>olqv.js: capabilities
+    olqv.js->>+serverWsgi: get fits header
+    serverWsgi->>olqv.js: header
+
+    alt path1
+        olqv.js->>+olqv.js: header declares 3D ? Yes
+        olqv.js->>+olqv.ejs: display spectrum
+    else path2:
+        olqv.js->>+olqv.js: header declares 2D ? Yes
+        olqv.js->>+olqv_2-5.ejs: display spectrum
+    else path2:
+        olqv.js->>+olqv.js: header declares another case ? Yes
+        olqv.js->>+olqv_error.ejs: display spectrum
+    end
+```
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