$ ng build -c demo
⠙ Generating browser application bundles (phase: setup)...Processing legacy "View Engine" libraries:
...
- angular-plotly.js [es2015/esm2015] ()
...
Encourage the library authors to publish an Ivy distribution.
✔ Browser application bundle generation complete.
✔ Copying assets complete.
✔ Index html generation complete.

$ ng version

     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/
    

Angular CLI: 13.2.2
Node: 14.17.0
Package Manager: npm 7.20.5
OS: linux x64

Angular: 13.2.1
... animations, cdk, common, compiler, compiler-cli, core, forms
... language-service, material, platform-browser
... platform-browser-dynamic, router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1302.2
@angular-devkit/build-angular   13.2.2
@angular-devkit/core            13.2.2
@angular-devkit/schematics      13.2.2
@angular/cli                    13.2.2
@angular/flex-layout            13.0.0-beta.36
@schematics/angular             13.2.2
rxjs                            6.6.7
typescript                      4.4.4

$ npm list angular-plotly.js
...
└── [email protected]

export declare class PlotlyViaCDNModule {
                           ~~~~~~~~~~~~~~~~~~
    This likely means that the library (angular-plotly.js) which declares PlotlyViaCDNModule has not been processed correctly by ngcc, or is not compatible with Angular Ivy. Check if a newer version of the library is available, and update if so. Also consider checking with the library's authors to see if the library is expected to be compatible with Ivy.