# Overview and Definition

A diagram is a visual representation of systems, structures, and relationships using graphs, charts, drawings, maps, sketches or schemas. It is a simple data visualization or graphical representation tool that shows the different parts or components of something and how they work and interact with each other.

There are many types of diagrams, such as flowcharts, circuits, E-R diagrams, UML diagrams, network diagrams, etc. and many ways to draw them.

# How to create a diagram with Dia

Dia is a free general-purpose diagramming software. It is a multiplatform application that supports more than 30 different types, such as flow charts, network diagrams, database models, UML, etc. It is a free alternative to Microsoft’s Visio.

1. Download it, install it, and launch it. It is a multiplatform application, so it is available for Windows, macOS, and GNU/Linux.
2. Create a new diagram: File, New.
3. File, Page Setup allows you to set the page size (A4, A3, Letter, etc.), orientation (portrait or landscape) and margins.
4. Let’s draw a flow chart. A flow chart is a type of diagram that represents or visualizes a process, workflow or algorithm. Let’s write an algorithm to find all the roots of a quadratic equation. 1. Input A, B, and C. 2. Calculate the discriminant of the quadratic equation: D = B^2 -4*A*C. 3. If the discriminant is negative, print an error message. If the discriminant is zero, then print the double root. Otherwise, it prints the two roots. Select Other sheets, Flow Chart in the diagram tools windows.
5. Draw the flow chart using the appropriate symbols for each element: Diamonds represent control decisions; rectangles (Process/Auxiliary Operation) are instructions, actions or calculations; ovals (Terminal interrupt) show where the algorithm begins and ends; the Manual input symbol (e.g., Read A, B, C) represents the manual input of data into one or more variables through a keyboard. Display indicates where and what information will be displayed to the user.
6. Align and spread out the components. Select all your components (Select, All or Ctrl+A), align all the components to the center (Object, Align, Center), left, or right, or spread out all the components vertically (Object, Align, Spread Out Vertically) or horizontally.
7. Connect your components using arrows. An arrow shows the direction of the process flow.
8. Save your diagram: File, Save.
9. Export the diagram as an image or pdf: File, Export. Select the file type as .png, .jpeg, .tiff, .pdf, etc.

# How to create a diagram with diagrams.net

diagrams.net is a free online diagram tool for making flowcharts, process diagrams, organization charts, UML, ER and network diagrams, among others.

1. Open your favorite browser, go to diagrams.net, and select the location where you want to store your diagrams, e.g., Google Drive, OneDrive, Dropbox, etc.

2. If you have selected a cloud storage platform like Google Drive, OneDrive or Dropbox, then you need to authorize diagrams.net in your Google Drive, OneDrive or Dropbox account respectively.

3. Click Create New Diagram to start creating a new diagram in the diagram.net editor.

4. Select a category (e.g., Basic, Network, Software, UML, Venn, etc.), choose a template from which you would like to create a new diagram (e.g., Class Diagram, Flowchart, Entity Relationship Diagram, etc.), then smash the Create button.

An Entity Relationship Diagram, also know as an ERD or an ER diagram, is a specialized graphic or diagram that represents relationships among entities in a database. There are three basic concepts: entities, attributes, and relationships.

5. Add, edit, and replace shapes and connectors using the Shape library. Observe that the relationship between Employee and Department is 0:Many and between Employee and Project is many-to-many.

This is because a project can be worked on by many employees and an employee can work on many projects, so we introduce a junction table (EmployerProject) into the model. We will set the EmployerProject’s primary key to include all the primary keys from the other two tables (employeeId, projectId) and define two one-to-many relationships between the two original tables (Employee and Project) and the new junction table (EmployeeProject). The junction table is at the “many” side of both relationships.

6. Click on the plus + symbol to add a new page to your diagram. Besides, you can save your diagram as an image or a SVG file (Export as, PNG…/JPEG…/SVG…) or you can embed your diagram into your web page (Export as…, HTML).

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