how do i take a generated value and insert into a graph?

Question

Hi I'm new to web designing and some of my jargon may be wrong. I'm trying to create a graph for lectures that shows the average rating each lecture gets. I have already calculated the average for each of the 18 lectures. Now I want to take the value for each one and insert it as the value in the graph.

Here is how I query and output my lectures:

<?php
    ini_set('display_errors', 1);
    include ('connection.php');
    $queryLecture = "SELECT DISTINCT lectureID FROM LectureReview ORDER BY lectureID";
    $resultLecture = $mysqli->query($queryLecture);

    while ($rowLecture = $resultLecture->fetch_assoc()) {
        echo "<div class=\"row\">";
        echo "<div class=\"box\">Lecture{$rowLecture['lectureID']}:</div>";
        $lectureID = $rowLecture['lectureID'];

        $mysqli2 = new mysqli($hostname, $username, $password, $database);
        $stmt = $mysqli2->prepare("SELECT AVG( lectureReview ) AS lectureAvg FROM LectureReview WHERE lectureID = ?");

        $stmt->bind_param('i', $lectureID);
        $stmt->execute(); 
        $stmt->bind_result($lectureAvg); 
        $stmt->fetch();

        echo "<div class=\"box\">Average Lecture Rating: {$lectureAvg}</div>";
        echo "</div>";
    }
?>

Then, OnLoad I do this:

var chart = new CanvasJS.Chart("chartContainer", {
    theme: "theme1",
    title:{
        text: "Average Lecture Score"              
    },
    animationEnabled: true,   
    data: [              
        {
            // Change type to "bar", "area", "spline", "pie",etc.
            type: "column",
            dataPoints: [
                { label: "Lecture 1", y: 4.5 },
                { label: "Lecture 2", y: 4.5 },
                { label: "Lecture 3", y: 5.0 },
                { label: "Lecture 4", y: 5.0 },
                { label: "Lecture 5", y: 5.0 },
                { label: "Lecture 6", y: 5.0 },
                { label: "Lecture 7", y: 5.0 },
                { label: "Lecture 8", y: 5.0 },
                { label: "Lecture 9", y: 5.0 },
                { label: "Lecture 10", y: 5.0 },
                { label: "Lecture 11", y: 5.0 },
                { label: "Lecture 12", y: 5.0 },
                { label: "Lecture 13", y: 5.0 },
                { label: "Lecture 14", y: 5.0 },
                { label: "Lecture 15", y: 5.0 },
                { label: "Lecture 16", y: 5.0 },
                { label: "Lecture 17", y: 5.0 },
                { label: "Lecture 18", y: 5.0 },
            ]
        }
    ]
});
chart.render();

And my target container:

<div id="chartContainer"
    style="height: 300px; width: 90%; position: absolute; padding-top:50px;">
</div>

At the moment it looks like this

How can I use the values from the database for the CanvasJS graph?

Answer 1

This is how I would do it:

Firstly, create an array outside your while loop. This is going to hold the total dataset for your graph. So:

$dataset = array();

Then, inside your while loop, create another array. This array will store the lecture name and its average rating. This array will have the associative names label and y (used in your chart dataset).

$lectureRating = array();

At the end of the loop, you then add the $lectureID and the $lectureAvg to that array. And then add it to the $dataset array.

$lectureRating['label'] = $lectureID;
$lectureRating['y'] = $lectureAvg;
array_push($dataset, $lectureRating);

After the while loop is finished the $dataset array should have a similar structure to:

Array[n] (
    [0] Array[2](
        ['label'] => "Lecture ID 1"
        ['y'] => 4.3
    )

    [1] Array[2](
        ['label'] => "Lecture ID 2"
        ['y'] => 3.7
    )

    // And so on
)

We now need to transform this into a Javascript Object (JSON) so that the js can interpret it. This is done by encoding it.

$graphData = json_encode($dataset);

This turns the above array into something like this (look familiar?):

[{"label": "Lecture ID 1", "y": 4.3}, {"label": "Lecture ID 2", "y": 3.7}]

This can then be output into the dataPoints object of your chart.

data: [
    {
        type: "column",
        dataPoints: <?php echo $graphData; ?>
    }