Android: RelativeLayout disables clickable Imageview button

in my android app, I use a simple screen with an MPandroidchart wrapped in a RelativeLayout and a back button at the top. The button does not respond.

The button (clickable imageview) does work as intended, if I change the RelativeLayout into LinearLayout. However, I do need RelativeLayout to get my MPAndroidchart working. Any Ideas how to make the button work?

THe button should be at the top left, above the chart.

XML below:

<?xml version="1.0" encoding="utf-8"?>

<layout xmlns:app="http://schemas.android.com/apk/res-auto">
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingBottom="16dp"
        android:paddingLeft="16dp"
        android:paddingRight="16dp"
        android:paddingTop="16dp"
        android:orientation="vertical"
        tools:context="com.example.erik.myroom.Frontend.Fragment_Statistic">

            <ImageView
                android:id="@+id/back_from_statistic"
                android:layout_width="32dp"
                android:layout_height="32dp"
                android:layout_weight="1"

                android:gravity="center_vertical|start"

                android:layout_marginTop="@dimen/material_drawer_vertical_padding"

                android:clickable="true"

                app:ico_color="@color/md_black_1000"
                app:ico_icon="@string/gmd_arrow_back"
                app:ico_size="16dp"/>

            <com.github.mikephil.charting.charts.HorizontalBarChart
                android:id="@+id/testchart"
                android:layout_width="match_parent"
                android:layout_height="match_parent" />

    </RelativeLayout>
</layout>

The ImageView is not disabled, but it is covered by the HorizontalBarChart.
Remove these attributes from the ImageView:

android:layout_weight="1"
android:gravity="center_vertical|start"

because they are valid only in a LinearLayout and set these attributes:

android:layout_alignParentStart="true"
android:layout_alignParentTop="true"

and also set this attribute:

android:layout_below="@id/back_from_statistic"

for the HorizontalBarChart.